supergrass

noun

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

   “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.”

   “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.”