supergrass

super- +‎ grass, from grass (“an informer”).

• (Received Pronunciation) IPA^(key): /ˈs(j)uːpə(ɹ)ˌɡɹɑːs/

supergrass (plural supergrasses)

1. (UK, slang) An informer providing substantial quantities of information on criminal or terrorist activities to the police.

   Synonyms: nark, nonce, snout; see also Thesaurus:informant

   • 1979 July 23, Havers, Michael (Attorney-General), “Witnesses (Immunity From Prosecution)”, in Hansard, volume 971, Parliament of the United Kingdom, page 64:

     I have to tell the hon. and learned Gentleman that I do not know the facts which led to that decision. On the other hand, although it is an unpleasant fact of life, there are now and again occasions on which, in order to achieve convictions against really bad villains, it is necessary to use people who are termed supergrasses.

   • 2000, D. Walsh, Bloody Sunday and the Rule of Law in Northern Ireland, page 241:

     In several cases the supergrass spent days in the witness box giving evidence of scores of offences allegedly committed by many individuals over a period of years.