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[rat] /ræt/
any of several long-tailed rodents of the family Muridae, of the genus Rattus and related genera, distinguished from the mouse by being larger.
any of various similar or related animals.
Slang. a scoundrel.
  1. a person who abandons or betrays his or her party or associates, especially in a time of trouble.
  2. an informer.
  3. a scab laborer.
Slang. a person who frequents a specified place:
a mall rat; gym rats.
a pad with tapered ends formerly used in women's hair styles to give the appearance of greater thickness.
rats, Slang. (an exclamation of disappointment, disgust, or disbelief.)
verb (used without object), ratted, ratting.
  1. to desert one's party or associates, especially in a time of trouble.
  2. to turn informer; squeal:
    He ratted on the gang, and the police arrested them.
  3. to work as a scab.
to hunt or catch rats.
verb (used with object), ratted, ratting.
to dress (the hair) with or as if with a rat.
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 of rat
before 1000; Middle English rat(t)e, Old English ræt; cognate with Dutch rat, German Ratz, Ratte
Related forms
ratlike, adjective Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2018.
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British Dictionary definitions for 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 See also brown rat, black rat
(informal) a person who deserts his or her friends or associates, esp in time of trouble
(informal) a worker who works during a strike; blackleg; scab
(slang, mainly US) an informer; stool pigeon
(informal) a despicable person
smell a rat, to detect something suspicious
verb rats, ratting, ratted
(informal) (intransitive) usually foll by on
  1. to divulge secret information (about); betray the trust (of)
  2. to default (on); abandon: he ratted on the project at the last minute
to hunt and kill rats
See also rats
Derived Forms
ratlike, adjective
Word Origin
Old English rætt; related to Old Saxon ratta, Old High German rato
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Word Origin and History for rat

late Old English ræt "rat," of uncertain origin. Similar words are found in Celtic (Gaelic radan), Romanic (Italian ratto, Spanish rata, French rat) and Germanic (Old Saxon ratta; Dutch rat; German Ratte, dialectal Ratz; Swedish råtta, Danish rotte) languages, but connection is uncertain and origin unknown. In all this it is very much like cat.

Perhaps from Vulgar Latin *rattus, but Weekley thinks this is of Germanic 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 Old English ræt to Latin rodere and thus PIE *red- "to scrape, scratch, gnaw," source of rodent (q.v.). Klein says there is no such connection and suggests a possible cognate in Greek 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 Romanic, but takes no position on ultimate origin.

RATS. Of these there are the following kinds: a black rat and a grey rat, a py-rat and a cu-rat. ["Dictionary of the Vulgar Tongue," Grose, 1788]
Middle English common form was ratton, from augmented Old French form raton. Sense of "one who abandons his associates" (1620s) 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 American English, 1886. To smell a rat is 1540s; "to be put on the watch by suspicion as the cat by the scent of a rat; to suspect danger" [Johnson]. _____-rat, "person who frequents _____" (in earliest reference dock-rat) is from 1864.


1812, "to desert one's party; 1864 as "to catch rats;" 1921 as "to peach on, inform on, behave dishonestly toward;" from rat (n.). Related: Ratted; ratting.

Online Etymology Dictionary, © 2010 Douglas Harper
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rat in Medicine

rat (rāt)
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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Slang definitions & phrases for rat


combining word

A frequenter and devotee of the place indicated: arcade rat/ rink rat (1970s+)


  1. A treacherous and disgusting person: He's acting like a prime rat on this (1629+)
  2. An informer; stool pigeon: In most cases they were ''rats'' and the best tools the keepers had (1902+)


  1. To betray; desert; turn one's coat (1812+)
  2. : an inmate, rankled by Angelo's attempts to woo his daughter, ratted on them (1910+)
  3. hoodge (1980s+)

Related Terms

brig rat, look like a drowned rat, pack rat, rug rat, shack man, smell a rat

The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D.
Copyright (C) 2007 by HarperCollins Publishers.
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Idioms and Phrases with rat
The American Heritage® Idioms Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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