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.
    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.
  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.


  1. rats, Slang. (an exclamation of disappointment, disgust, or disbelief.)

verb (used without object), rat·ted, rat·ting.

  1. Slang.
    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.
  2. to hunt or catch rats.

verb (used with object), rat·ted, rat·ting.

  1. to dress (the hair) with or as if with a rat.
  1. 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.


  1. an exclamation of rejection or disdain


  1. Australian slang deranged; insane


  1. 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 worldSee also brown rat, black rat
  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, mainly US an informer; stool pigeon
  5. informal a despicable person
  6. smell a rat to detect something suspicious

verb rats, ratting or ratted

  1. (intr usually foll by on) informal
    1. to divulge secret information (about); betray the trust (of)
    2. to default (on); abandonhe ratted on the project at the last minute
  2. to hunt and kill rats

expressing incredulity, disappointment, etc., 1886, from rat (n.).


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.


  1. 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.

In addition to the idioms beginning with rat

  • rat on
  • rat race

also see:

  • like a drowned rat
  • smell a rat
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