noun \ˈrat\

: a small animal that has a pointed nose and a long, thin tail

: a person who is not loyal or who cannot be trusted

: a person who is bad or cruel

Full Definition of RAT

a :  any of numerous rodents (Rattus and related genera) differing from the related mice especially by considerably larger size
b :  any of various similar rodents
:  a contemptible person: as
a :  one who betrays or deserts friends or associates
b :  scab 3b
c :  informer 2
:  a pad over which a woman's hair is arranged
:  a person who spends much time in a specified place <a mall rat>
rat·like \-ˌlīk\ adjective

Examples of RAT

  1. a dirty old building infested by rats and mice
  2. I can't believe that rat turned us in to the police!
  3. No one understands why she's with a rat like him.
  4. Every night he goes to work out with the other gym rats.

Origin of RAT

Middle English, from Old English ræt; akin to Old High German ratta rat and perhaps to Latin rodere to gnaw — more at rodent
First Known Use: before 12th century



: to tell someone in authority (such as the police) about something wrong that someone has done : to betray someone ( US )


Full Definition of RAT

intransitive verb
:  to betray, desert, or inform on one's associates —usually used with on
:  to catch or hunt rats
:  to work as a scab
transitive verb
:  to give (hair) the effect of greater quantity (as by use of a rat)
:  to inform on :  turn in —usually used with out <ratted out his accomplice>

Examples of RAT

  1. The teacher knows what we did, which means that somebody ratted.

First Known Use of RAT



noun \ˈrat\   (Medical Dictionary)

Medical Definition of RAT

: any of the numerous rodents (family Muridae) of Rattus and related genera that differ from the murid mice by their usually considerably larger size and by features of the teeth and other structures and that include forms (as the brown rat, the black rat, and the roof rat) which live in and about human habitations and in ships, have become naturalized by commerce in most parts of the world, and are destructive pests consuming or destroying vast quantities of food and other goods and acting as vectors of various diseases (as bubonic plague)


noun    (Concise Encyclopedia)

Norway rat (Rattus norvegicus).—John H. Gerard

Any of more than 500 forms of Asian rodent (genus Rattus, family Muridae) that have been introduced worldwide. The black rat (Rattus rattus) and the Norway rat (R. norvegicus) are the aggressive, omnivorous animals commonly associated with the name. They prefer areas of human habitation, where they can easily find food. They have keen senses and can climb, jump, burrow, or gnaw their way into seemingly inaccessible places. They reproduce extremely rapidly (up to 150 offspring a year) and have few natural predators. Rats transmit numerous human diseases and have often destroyed grain supplies. The black rat is about 8 in. (20 cm) long, excluding the slightly longer tail. The Norway rat (also called the brown, barn, sewer, or wharf rat) has proportionately smaller ears and a shorter tail. Laboratory rats are strains of the Norway rat. The name rat is applied, without scientific basis, to other rodents (e.g., kangaroo rat, wood rat).


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