grass (someone up)

Definition

Slang

  • verb to inform on or betray (someone) to the police or authorities. The usage was originally to ‘grass on someone’ or to ‘grass to the authorities’. From the 1980s the London underworld expression to ‘grass someone up’ has been used, not least among schoolchildren, who had adopted it from TV police dramas and documentaries. For the etymology see the noun grass.
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