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super- +‎ grass, from grass, British/Irish slang word for an informer.


supergrass (plural supergrasses)

  1. An informer providing substantial quantities of information on criminal or terrorist activities to the police.
    • 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.