rat

Concise Oxford English Dictionary © 2008 Oxford University Press:
rat/rat/

▶noun

• 1 a rodent resembling a large, long-tailed mouse, typically considered a serious pest. [Rattus norvegicus (brown rat), R. rattus (black or ship rat), and many other species.]

• 2 informal a despicable person.

  ■ an informer.

• 3 N. Amer. informal a person who is associated with or frequents a specified place: a mall rat.

▶exclamation (rats) informal expressing mild annoyance. ▶verb (rats, ratting, ratted)

• 1 hunt or kill rats.

• 2 informal desert one's party, side, or cause.

  ■ (rat on) break (an agreement or promise).

• 3 (rat on or N. Amer. also rat someone out) informal inform on (someone).

– derivatives
ratting noun.

– origin OE ræt, prob. of Romance origin; reinforced in ME by OFr. rat.