(UK, slang) To betray by informing on.
What I was pissed off about, though, was him telling everyone that he'd only grassed on me because I'd grassed on him first, which was a total lie.
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Regional slang from around the English-speaking world — British, Australian, Irish, Caribbean, Nigerian, Filipino, AAVE, and the hyphenated-English dialects that make the internet sound local.
See all Regional & Other slang on Slangora.