Past tense of 'grass': to inform on someone to authorities (British slang), or to cover with grass; to feed livestock on grass.
From Old English 'græs.' The 'inform on' meaning is British slang, possibly from the idea of 'grassing over' or covering something up by revealing it.
In British crime slang, 'grassed' means you informed the police about someone—it's a serious accusation that means you betrayed your associates!
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