1 [bak]
the rear part of the human body, extending from the neck to the lower end of the spine.
the part of the body of animals corresponding to the human back.
the rear portion of any part of the body: the back of the head.
the whole body, with reference to clothing: the clothes on his back.
ability for labor; effort; endurance: He put his back into the task.
the part opposite to or farthest from the front; the rear part: the back of a hall.
the part that forms the rear of any object or structure: the back of a chair.
the part that covers the back: the back of a jacket.
the spine or backbone: The fall broke his back.
any rear part of an object serving to support, protect, etc.: the back of a binder.
Nautical, Aeronautics. the forward side of a propeller blade (opposed to face ).
Aeronautics. the top part or upper surface of an aircraft, especially of its fuselage.
Bookbinding. the edge of a book formed where its sections are bound together.
the backs, grounds along the River Cam in back of certain colleges at Cambridge University in England: noted for their great beauty.
Architecture, extrados.
the upper side of a joist, rafter, handrail, etc.
the area of interior wall between a window stool and the floor.
Mining. the roof of a stope or drift.
a player whose regular position is behind that of players who make initial contact with the opposing team, as behind the forward line in football or nearest the player's own goal in polo.
the position occupied by this player.
verb (used with object)
to support, as with authority, influence, help, or money (often followed by up ): to back a candidate; to back up a theory with facts.
to bet on: to back a horse in the race.
to cause to move backward (often followed by up ): to back a car.
to furnish with a back: to back a book.
to lie at the back of; form a back or background for: a beach backed by hills.
to provide with an accompaniment: a singer backed by piano and bass.
to get upon the back of; mount.
to write or print on the back of; endorse; countersign.
Carpentry. to attach strips of wood to the upper edge of (a joist or rafter) to bring it to a desired level.
to alter the position of (a sail) so that the wind will strike the forward face.
to brace (yards) in backing a sail.
to reinforce the hold of (an anchor) by means of a smaller one attached to it and dropped farther away.
verb (used without object)
to go or move backward (often followed by up ).
Nautical. (of wind) to change direction counterclockwise (opposed to veer ).
situated at or in the rear: at the back door; back fence.
far away or removed from the front or main area, position, or rank; remote: back settlements.
belonging to the past: back files; back issues.
in arrears; overdue: back pay.
coming or going back; moving backward: back current.
Navigation, reciprocal ( def 7 ).
Phonetics. (of a speech sound) produced with the tongue articulating in the back part of the mouth, as in either of the sounds of go.
Verb phrases
back away, to retreat; withdraw: They gradually began to back away from their earlier opinion.
back down, to abandon an argument, opinion, or claim; withdraw; retreat: He backed down as soon as a member of the audience challenged his assertion.
back off,
to back down: Now that the time for action had arrived, it was too late to back off.
Textiles. to reverse (the spindle) in mule spinning prior to winding on the newly spun length of yarn.
back out (of), to fail to keep an engagement or promise; withdraw from; abandon: Two entrants have backed out of competing in the marathon. You can't back out now.
back up,
to bring (a stream of traffic) to a standstill: A stalled car backed up traffic for miles.
Printing. to print a sheet again on its other side.
Printing. to fill in (the thin copper shell of an electrotype) with metal in order to strengthen it.
to move backward: Back up into the garage.
to reinforce: We backed up the cardboard with slats so it wouldn't fall down.
to support or confirm: He backed up my story and they let us go.
Computers. to duplicate (a file or a program) as a precaution against failure.
back up for, Australian Informal. to return for more of, as another helping of food.
back and fill,
Nautical. to trim the sails of a boat so that the wind strikes them first on the forward and then on the after side.
to change one's opinion or position; vacillate.
back and forth, South Midland U.S.
to go back and forth, as in running errands or visiting: He spent the day backing and forthing to the post office.
to work in an aimless or ineffective way; expend effort with little result.
back water,
Nautical. to reverse the direction of a vessel.
to retreat from a position; withdraw an opinion: I predict that the council will back water on the tax issue.
be flat on one's back,
to be helpless or beaten: He's flat on his back after a long succession of failures.
to be confined to one's bed because of illness.
behind one's back, in one's absence; without one's knowledge; treacherously; secretly: I'd rather talk to him about it directly than discuss it behind his back.
break someone's back, to cause a person to fail, especially to cause to become bankrupt: His family's extravagance is breaking his back.
break the back of,
to complete the principal or hardest part of (a project, one's work, etc.): He finally broke the back of the problem.
to overcome; defeat: They broke the back of our union.
get off one's back, Informal. to cease to find fault with or to disturb someone: The fight started when they wouldn't get off my back.
get one's back up, Informal. to become annoyed; take offense: She gets her back up whenever someone mentions her family's influence.
get / have / watch someone’s back, Informal. to help and protect someone if necessary, especially in a time of trouble: If he needs anything, I hope he knows I’ve got his back. Also, have got someone's back.
have one's back to the wall, to be in a difficult or hopeless situation.
in back of, behind: He hid in back of the billboard. What could be in back of his strange behavior? Also, back of.
on one's back, Informal. finding fault with or disturbing someone: The boss is always on my back about promptness.
pat on the back. pat1 ( defs 9, 11 ).
a stab in the back. stab ( def 12 ).
stab someone in the back. stab ( def 13 ).
turn one's back on,
to forsake or neglect: He was unable to turn his back on any suffering creature.
to leave behind, as in anger.

before 1000; Middle English bak, Old English bæc back of the body; cognate with Old Frisian bek, Old Saxon, Old Norse bak; perhaps < Indo-European *bhogo- bending; cf. bacon

backless, adjective

back up, backup.

19. sustain, abet, favor, assist; countenance, endorse. 29. retire, retreat, withdraw. 31. Back, hind, posterior, rear refer to something situated behind something else. Back means the opposite of front: back window. Hind, and the more formal word posterior, suggest the rearmost of two or more often similar objects: hind legs; posterior lobe. Rear is used of buildings, conveyances, etc., and in military language it is the opposite of fore: rear end of a truck; rear echelon.

1, 31. front.

55. Although some object to their use, the phrases in back of and the shorter—and much older—back of with the meaning “behind” are fully established as standard in American English: The car was parked (in) back of the house. Both phrases occur in all types of speech and writing. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2015.
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World English Dictionary
back1 (bæk)
1.  the posterior part of the human body, extending from the neck to the pelvisRelated: dorsal
2.  the corresponding or upper part of an animal
3.  the spinal column
4.  the part or side of an object opposite the front
5.  the part or side of anything less often seen or used: the back of a carpet; the back of a knife
6.  the part or side of anything that is furthest from the front or from a spectator: the back of the stage
7.  the convex part of something: the back of a hill; the back of a ship
8.  something that supports, covers, or strengthens the rear of an object
9.  ball games
 a.  a mainly defensive player behind a forward
 b.  the position of such a player
10.  the part of a book to which the pages are glued or that joins the covers
11.  mining
 a.  the side of a passage or layer nearest the surface
 b.  the earth between that level and the next
12.  Compare bed the upper surface of a joist, rafter, slate, tile, etc, when in position
13.  at one's back behind, esp in support or pursuit
14.  at the back of one's mind not in one's conscious thoughts
15.  behind one's back without one's knowledge; secretly or deceitfully
16.  break one's back to overwork or work very hard
17.  break the back of to complete the greatest or hardest part of (a task)
18.  on one's back, flat on one's back incapacitated, esp through illness
19.  informal get off someone's back to stop criticizing or pestering someone
20.  have on one's back to be burdened with
21.  informal on someone's back criticizing or pestering someone
22.  put one's back into to devote all one's strength to (a task)
23.  put someone's back up, get someone's back up to annoy someone
24.  see the back of to be rid of
25.  back of beyond
 a.  the back of beyond a very remote place
 b.  (Austral) in such a place (esp in the phrase out back of beyond)
26.  turn one's back on
 a.  to turn away from in anger or contempt
 b.  to refuse to help; abandon
27.  with one's back to the wall in a difficult or desperate situation
28.  (also intr) to move or cause to move backwards
29.  to provide support, money, or encouragement for (a person, enterprise, etc)
30.  to bet on the success of: to back a horse
31.  to provide with a back, backing, or lining
32.  to provide with a music accompaniment: a soloist backed by an orchestra
33.  to provide a background for; be at the back of: mountains back the town
34.  to countersign or endorse
35.  archaic to mount the back of
36.  (intr; foll by on or onto) to have the back facing (towards): the house backs onto a river
37.  (intr) See veer (of the wind) to change direction in an anticlockwise direction in the northern hemisphere and a clockwise direction in the southern
38.  nautical to position (a sail) so that the wind presses on its opposite side
39.  back and fill
 a.  nautical to manoeuvre the sails by alternately filling and emptying them of wind to navigate in a narrow place
 b.  to vacillate in one's opinion
40.  situated behind: a back lane
41.  of the past: back issues of a magazine
42.  owing from an earlier date: back rent
43.  chiefly (US), (Austral), (NZ) remote: back country
44.  (of a road) not direct
45.  moving in a backward direction: back current
46.  phonetics of, relating to, or denoting a vowel articulated with the tongue retracted towards the soft palate, as for the vowels in English hard, fall, hot, full, fool
47.  at, to, or towards the rear; away from something considered to be the front; backwards; behind
48.  in, to, or towards the original starting point, place, or condition: to go back home; put the book back; my headache has come back
49.  in or into the past: to look back on one's childhood
50.  in reply, repayment, or retaliation: to hit someone back; pay back a debt; to answer back
51.  in check: the dam holds back the water
52.  in concealment; in reserve: to keep something back; to hold back information
53.  back and forth to and fro
54.  back to front
 a.  in reverse
 b.  in disorder
Related: dorsal
[Old English bæc; related to Old Norse bak, Old Frisian bek, Old High German bah]

back2 (bæk)
a large tub or vat, esp one used by brewers
[C17: from Dutch bak tub, cistern, from Old French bac, from Vulgar Latin bacca (unattested) vessel for liquids]

back up
vb (usually foll by on)
1.  (tr) to support or assist
2.  (intr) cricket (of a nonstriking batsman) to move down the wicket in readiness for a run as a ball is bowled
3.  (of water) to accumulate
4.  (of traffic) to become jammed behind an accident or other obstruction
5.  computing to make a copy of (a data file), esp for storage in another place as a security copy
6.  printing to print the second side of (a sheet)
7.  (Austral) to repeat an action immediately
8.  a support or reinforcement
9.  a.  a reserve or substitute
 b.  (as modifier): backup troops
10.  (US), (Canadian)
 a.  musical accompaniment, esp for a pop singer
 b.  (as modifier): backup singer
11.  the overflow from a blocked drain or pipe
12.  computing a file or set of files copied for security purposes

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

O.E. bæc "back, backwards, behind," from P.Gmc. *bakam (cf. O.S., M.Du. bak, O.Fris. bek), with no known connections outside Germanic. The cognates mostly have been ousted in this sense in other modern Gmc. languages by words akin to Modern English ridge (cf. Dan. ryg,
Ger. Rücken). Many I.E. languages show signs of once having distinguished the horizontal back of an animal (or a mountain range) from the upright back of a human. In other cases, a modern word for "back" may come from a word related to "spine" (It. schiena, Rus. spina) or "shoulder, shoulder blade" (Sp. espalda, Pol. plecy).

back up
1767, "stand behind and support," from back (v.) + up. The noun meaning "standby, reserve" is recorded from 1952 (often written as one word, backup); specific reference to computing is from 1965.

late 15c., "to move (something) back," from back (adj.); meaning "to support" (as by a bet) is first attested 1540s.

O.E. bæc "backwards, behind" (see back (n.)). Back-seat driver first attested 1926. The back of (one's) hand has been used to imply contempt and rejection since at least 1300; to know (something) like the back of one's hand, implying familiarity, is first attested 1943.
To be on the back burner in the figurative sense is from 1960, from the image of a cook keeping a pot there to simmer while he or she works on another concoction at the front of the stove. Back-to-nature (adj.) is first attested 1915.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

back (bāk)

  1. The posterior portion of the trunk of the human body between the neck and the pelvis; the dorsum.

  2. The backbone or spine.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Idioms & Phrases

back up

  1. Move or drive a vehicle backward, as in He told her to back up into the garage. [First half of 1800s]

  2. Bring or come to a standstill, as in The water had backed up in the drains, or The accident had backed up traffic for miles. [First half of 1800s]

  3. Support or strengthen, as in The photos were backed up with heavy cardboard so they couldn't be bent, or I'll back up that statement of yours. [Second half of 1700s]

  4. Duplicate a file or program so that the original is not lost. For example, Every computer manual warns you to back up your work frequently in case of a power outage or computer failure. [Second half of 1900s]

The American Heritage® Dictionary of Idioms by Christine Ammer.
Copyright © 1997. Published by Houghton Mifflin.
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Example sentences
All you do is move the question back up the line of how did life come into
We are finally settled-in having moved back up the north of the state.
Past that wonderfully distinctive gate--scan back up and take a closer look at
  it--things get even more interesting.
Now he had a bodily frame which could back up any resolution he might take.
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