The act of covering with grass or seeding an area with grass; also slang for informing on someone to authorities.
Present participle of 'grass' (Old English 'gærs' used as a verb), plus '-ing' (Old English '-ing'). The slang meaning emerged in British English in the 20th century, possibly from rhyming slang or extended meaning.
The dual meaning of 'grassing'—both agricultural (seeding grass) and criminal slang (informing/betrayal)—shows how the same word can develop completely different meanings in different communities; British criminals likely chose 'grass' for the rhyming slang connection 'grass→grass in one's mouth' (shut your mouth).
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