verb (used without object), flapped, flapping.
to swing or sway back and forth loosely, especially with noise: A loose shutter flapped outside the window.
to move up and down, as wings; flap the wings, or make similar movements.
to strike a blow with something broad and flexible.
Slang. to become excited or confused, especially under stress: a seasoned diplomat who doesn't flap easily.
verb (used with object), flapped, flapping.
to move (wings, arms, etc.) up and down.
to cause to swing or sway loosely, especially with noise.
to strike with something broad and flat.
to toss, fold, shut, etc., smartly, roughly, or noisily.
Phonetics. to pronounce (a sound) with articulation resembling that of a flap: The British often flap their r's.
something flat and broad that is attached at one side only and hangs loosely or covers an opening: the flap of an envelope; the flap of a pocket.
either of the two segments of a book jacket folding under the book's front and back covers.
one leaf of a folding door, shutter, or the like.
a flapping motion.
the noise produced by something that flaps.
a blow given with something broad and flat.
a state of nervous excitement, commotion, or disorganization.
an emergency situation.
scandal; trouble.
Surgery. a portion of skin or flesh that is partially separated from the body and may subsequently be transposed by grafting.
Aeronautics. a movable surface used for increasing the lift or drag of an airplane.
a rapid flip of the tongue tip against the upper teeth or alveolar ridge, as in the r -sound in a common British pronunciation of very, or the t -sound in the common American pronunciation of water.
a trill.
a flipping out of the lower lip from a position of pressure against the upper teeth so as to produce an audible pop, as in emphatic utterances containing f -sounds or v -sounds.
Building Trades.
Also called backflap hinge, flap hinge. a hinge having a strap or plate for screwing to the face of a door, shutter, or the like. See illus. under hinge.
one leaf of a hinge.

1275–1325; Middle English flappe a blow, slap, flappen to hit, slap; compare Dutch flap, flappen

flapless, adjective
unflapping, adjective
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
flap (flæp)
vb (sometimes foll by down) , flaps, flapping, flapped
1.  to move (wings or arms) up and down, esp in or as if in flying, or (of wings or arms) to move in this way
2.  to move or cause to move noisily back and forth or up and down: the curtains flapped in the breeze
3.  informal (intr) to become agitated or flustered; panic
4.  to deal (a person or thing) a blow with a broad flexible object
5.  to toss, fling, slam, etc, abruptly or noisily
6.  (tr) phonetics to pronounce (an (r) sound) by allowing the tongue to give a single light tap against the alveolar ridge or uvula
7.  the action, motion, or noise made by flapping: with one flap of its wings the bird was off
8.  a piece of material, etc, attached at one edge and usually used to cover an opening, as on a tent, envelope, or pocket
9.  a blow dealt with a flat object; slap
10.  a movable surface fixed to the trailing edge of an aircraft wing that increases lift during takeoff and drag during landing
11.  surgery a piece of tissue partially connected to the body, either following an amputation or to be used as a graft
12.  informal a state of panic, distress, or agitation
13.  phonetics an (r) produced by allowing the tongue to give a single light tap against the alveolar ridge or uvula
[C14: probably of imitative origin]

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

early 14c., flappe "a blow, slap," probably imitative of the sound of striking. Meaning "something that hangs down" is first recorded 1520s. Sense of "motion or noise like a bird's wing" is 1774; meaning "disturbance, noisy tumult" is 1916, British slang. The verb meaning "to swing loosely" is from 1520s.
Related: Flapped; flapping.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

flap (flāp)
Tissue used in surgical grafting that is only partially detached from its donor site so that it continues to be nourished during transfer to the recipient site.

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

FLAP definition

A symbolic mathematics package for IBM 360.
["FLAP Programmer's Manual", A.H. Morris Jr., TR-2558 (1971) US Naval Weapons Lab].
[Sammet 1969, p. 506].
[Jargon File]

flap definition

1. To unload a DECtape (so it goes flap, flap, flap). Old-time hackers at MIT tell of the days when the disk was device 0 and microtapes were 1, 2, etc. and attempting to flap device 0 would instead start a motor banging inside a cabinet near the disk.
The term is used, by extension, for unloading any magnetic tape. See also macrotape. Modern cartridge tapes no longer actually flap, but the usage has remained.
The term could well be re-applied to DEC's TK50 cartridge tape drive, a spectacularly misengineered contraption which makes a loud flapping sound, almost like an old reel-type lawnmower, in one of its many tape-eating failure modes.
2. See flapping router.
[Jargon File]
The Free On-line Dictionary of Computing, © Denis Howe 2010 http://foldoc.org
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Example sentences
The flap of the envelope should be plain and the point not unduly long.
The beasts trumpet loudly, flap their ears and turn in circles.
When hemorrhaging occurs on either flap, the normally flexible folds swell and
  impair proper functioning.
Once it nears the surface again, it can flap its tail and taxi without fully
  returning to the water.
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