From grass (“informer”) + up.
grass up (third-person singular simple present grasses up, present participle grassing up, simple past and past participle grassed up)
- (Britain, slang) To betray by informing on.
- 1999, Angela Devlin, Bob Turney, Going Straight: After Crime and Punishment →ISBN:
- My sister grassed me up to the caretaker! She thought she was doing me a favour — by grassing me up she was sure he'd let me off.
- ^ The New Partridge Dictionary of Slang →ISBN