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1. slang An informant who gives evidence against their associate(s) or accomplice(s) in exchange for immunity or reduced sentencing, especially in high-profile or large-scale criminal trials. Primarily heard in UK. The supergrass was facing over 30 felony charges, but he ended up only receiving a two-year prison sentence because of all the evidence he had supplied Scotland Yard about his criminal associates over the years. Little did we know that the mild-manner Mr Collins next door was really a supergrass who had been placed in witness protection.
2. slang Marijuana laced with phencyclidine (PCP). Be careful—he's high on supergrass and could turn violent on you! I only tried supergrass one time in college, but I got so out of my mind that I thought I would never feel normal again. Needless to say, I haven't touched the stuff since.
n. high-quality marijuana. The cannabis that is the richest in resin is sometimes called “supergrass.”