rat
n. countablen. an animal that looks like a large mouse and is often considered a pest. It's also a very strong insult for someone who betrays their friends or tells secrets to an enemy.
n. A medium-sized, long-tailed rodent, especially of the genus *Rattus*. Figuratively, it refers to a person who betrays a group or acts as an informant; this sense is highly pejorative and informal.
A rat runs across the subway tracks.
He was called a rat by his old friends after he testified against them in court.
The novel's protagonist is a cornered rat, lashing out at a society that has offered him nothing but contempt and the constant threat of exposure.
From Middle English ratte, rat, rotte, from Old English rætt, from Proto-West Germanic ratt, from Proto-Germanic rattaz, rattō (compare West Frisian rôt, Dutch rat), of uncertain origin, possibly from Proto-Indo-European Hreh₃d- (“to scrape, scratch, gnaw”). However, the rat may have been unknown in Northern Europe in antiquity, and the Proto-Germanic word may have referred to a different animal; see *rattaz for more. Attestation of this family of words begins in the 12th century. Some of the Germanic cognates show considerable consonant variation, e.g. Middle Low German ratte, radde; Middle High German rate, ratte, ratze. The irregularity may be symptomatic of a late dispersal of the word, although Kroonen accounts for it with a Proto-Germanic stem raþō nom., ruttaz gen., showing both ablaut and a Kluge's law alternation, with the variation arising from varying remodellings in the descendants. Kroonen states that this requires a Proto-Indo-European etymon in final t and is incompatible with the usual derivation from Proto-Indo-European Hreh₃d- (“to scrape, scratch, gnaw”).
From Middle English ratten, further etymology unknown. Compare Middle Low German retten (“to tear, tear up”), Middle High German ratzen (“to scratch; rasp; tear”). Could be related to write. See also rit.
He ratted his gang.He ratted on his gang.When used as a verb meaning 'to betray', 'rat' is intransitive and requires the preposition 'on'.