From Longman Dictionary of Contemporary Englishsupergrasssu‧per‧grass /ˈsuːpəɡrɑːs $ -pərɡræs/ noun [countable] British English informalSCCSCP a criminal who gives the police information about many other criminals, in order to get a less severe punishment
Examples from the Corpus
supergrass• From 1993 the state hit back with troops, new laws and supergrasses.• The report, drawn from evidence given by supergrasses, identified those whom the Mafia considered allies or potential allies.• And almost 10 years ago Atcheson was arrested and charged on the evidence of loyalist supergrass James Crockard.• Four of those are of supergrasses.• Local bully Billy Hodge is about to turn supergrass under pressure from his wife.• Both turned supergrass and were the main prosecution witnesses in the trial of the other men.• Prosecutors said the allegation was made long ago by an unreliable supergrass, and no investigation is expected. 