grass up - Wiktionary, the free dictionary

From grass (“informer”)^[1] + up.

grass up (third-person singular simple present grasses up, present participle grassing up, simple past and past participle grassed up)

1. (UK, 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.

• drop a dime, name names, snitch; See also Thesaurus:rat out

• supergrass