A supergrass is a criminal who tells the police about a large number of other criminals, usually in exchange for a lighter sentence or protection. This is strongly British — an American would say 'informant' or, informally, 'rat' or 'snitch'. It is journalists' and police vocabulary above all: you meet it in newspapers and crime reports rather than in ordinary conversation. The key to the word is 'grass', old British slang for an informer, someone who tells on others; 'to grass someone up' means to report them. Adding super- signals scale — not one tip-off but testimony that brings down a whole gang. The word carries a heavy load of judgement, and which way it leans depends on who is speaking: to the police a supergrass is a valuable witness, to the criminal world a traitor. It became especially common in British reporting on organised crime and on the conflict in Northern Ireland, where supergrass trials were a real and controversial thing.

（提供大量同夥情報以換取減刑的）污點證人、線人；台灣新聞多用「污點證人」，注意英文這個字帶有濃厚的英國背叛意味