See rat in Webster's New World Roget's A-Z Thesaurus II
One who betrays:
betrayer, double-crosser, Judas, traitor. See trustOne who gives incriminating information about others:
informant, informer, tattler, tattletale. (Informal) tipster. (Slang) fink, snitch, snitcher, squealer, stoolie, stool pigeon. See knowledge, lawA person who has defected:
apostate, defector, deserter, recreant, renegade, runagate, tergiversator, turncoat. See approachTo abandon one's cause or party usually to join another:
apostatize, defect, desert3, renegade, tergiversate, turn. Idioms: change sides, turn one's coat. See approach, trustTo be treacherous to.
Also used with on: betray, double-cross. (Slang) sell out. Idiom: sell down the river . See trustTo give incriminating information about others, especially to the authorities:
inform, talk, tattle, tip3 (off). (Informal) fink. (Slang) sing, snitch, squeal, stool. Idiom: blow the whistle. See knowledge, lawLearn more about rat