rat

[rat]
noun
1.
any of several long-tailed rodents of the family Muridae, of the genus Rattus and related genera, distinguished from the mouse by being larger.
2.
any of various similar or related animals.
3.
Slang. a scoundrel.
4.
Slang.
a.
a person who abandons or betrays his or her party or associates, especially in a time of trouble.
b.
an informer.
c.
a scab laborer.
5.
Slang. a person who frequents a specified place: a mall rat; gym rats.
6.
a pad with tapered ends formerly used in women's hair styles to give the appearance of greater thickness.
interjection
7.
rats, Slang. (an exclamation of disappointment, disgust, or disbelief.)
verb (used without object), ratted, ratting.
8.
Slang.
a.
to desert one's party or associates, especially in a time of trouble.
b.
to turn informer; squeal: He ratted on the gang, and the police arrested them.
c.
to work as a scab.
9.
to hunt or catch rats.
verb (used with object), ratted, ratting.
10.
to dress (the hair) with or as if with a rat.
Idioms
11.
smell a rat, to suspect or surmise treachery; have suspicion: After noting several discrepancies in his client's story, the attorney began to smell a rat.

Origin:
before 1000; Middle English rat(t)e, Old English ræt; cognate with Dutch rat, German Ratz, Ratte

ratlike, adjective
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
rat (ræt)
 
n
1.  brown rat See also black rat any of numerous long-tailed murine rodents, esp of the genus Rattus, that are similar to but larger than mice and are now distributed all over the world
2.  informal a person who deserts his or her friends or associates, esp in time of trouble
3.  informal a worker who works during a strike; blackleg; scab
4.  slang chiefly (US) an informer; stool pigeon
5.  informal a despicable person
6.  smell a rat to detect something suspicious
 
vb (usually foll by on) , rats, ratting, ratted
7.  informal
 a.  to divulge secret information (about); betray the trust (of)
 b.  to default (on); abandon: he ratted on the project at the last minute
8.  to hunt and kill rats
 
[Old English rætt; related to Old Saxon ratta, Old High German rato]
 
'ratlike
 
adj

Collins English Dictionary - Complete & Unabridged 10th Edition
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Etymonline
Word Origin & History

rat
O.E. ræt. Similar words in Celtic (Gael. radan), Romance (It. ratto, Sp. rata, Fr. rat) and Gmc. (M.L.G. rotte, Ger. ratte) languages, but connection is uncertain and origin unknown. Perhaps from V.L. *rattus, but Weekley thinks this is of Gmc. origin, "the animal having come from the East with
the race-migrations" and the word passing thence to the Romanic languages. American Heritage and Tucker connect O.E. ræt to L. rodere and thus PIE *red- "to scrape, scratch, gnaw," source of rodent (q.v.). Klein says there is no connection and suggests a possible cognate in Gk. rhine "file, rasp." Weekley connects them with a question mark and Barnhart writes, "the relationship to each other of the Germanic, Romance, and Celtic words for rat is uncertain." OED says "probable" the rat word spread from Germanic to Romance, but takes no position on ultimate origin. M.E. common form was ratton, from augmented O.Fr. form raton. Sense of "one who abandons his associates" (1629) is from belief that rats leave a ship about to sink or a house about to fall and led to meaning "traitor, informant" (1902; verb 1910). Interjection rats is Amer.Eng., 1886. To smell a rat is c.1550. Rat-race "competitive struggle" is 1939. Ratsbane (1523) is arsenic. Rat fink is teen slang from 1963. Rathole in fig. sense of "nasty, messy place" first attested 1812. _____-rat, "person who frequents _____" (in earliest ref. dock-rat) is from 1864. Rat-pack "juvenile gang" is from 1951.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

rat (rāt)
n.
Any of various long-tailed rodents of the genus Rattus and related genera, including certain strains used in scientific research and certain species that are vectors for various diseases.

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
Abbreviations & Acronyms
RAT
right anterior thigh
The American Heritage® Abbreviations Dictionary, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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American Heritage
Idioms & Phrases

rat

In addition to the idioms beginning with rat, also see like a drowned rat; smell a rat.

The American Heritage® Dictionary of Idioms by Christine Ammer.
Copyright © 1997. Published by Houghton Mifflin.
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Example sentences for +rat
Other animals such as the blind mole rat are truly blind and rely on other
  senses.
Dwarfs are also known to eat dog, but only if there is not any rat.
He battles a large rat and it inspires him with a plan, involving the corpse of
  the rat.
Apparatus for chronic stimulation of the brain of the rat by radiofrequency
  transmission.
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