supergrass.

About supergrass

Supergrass is a British slang term for an informant who turns King's evidence, often in return for protection and immunity from prosecution. In the British criminal world, police informants have been called "grasses" since the late 1930s, and the "super" prefix was coined by journalists in the early 1970s to describe those who gave evidence against fellow criminals in a series of high-profile mass trials at the time.