A Grass or 'Supergrass' is a slang term for an informer, which originated in London.
Informers had been referred to as grasses since the late-1930s, and the "super" prefix was coined by journalists in the early 1970s to describe those informers from the city's underworld who testified against former associates in a series of high-profile mass trials at the time.
Informants offer authorities a supply of information without the consent of the other parties (criminals or inmates) with the intent of malicious, personal or financial gain.
Informants are regarded as traitors by their former criminal associates. Whatever the nature of a group, it is bound to feel strong hostility towards any known informers, regard them as threats and inflict punishments ranging from social ostracism through physical abuse and/or death. Informers are therefore generally protected, either by being segregated while in prison or, if they are not incarcerated, relocated under a new identity.