grass on someone
phrasal verb with grass verb uk/ɡrɑːs/ us/ɡræs/
UK slang
向警方告密，向警方告发（某人）
If a person grasses on someone else, they tell the police or someone in authority about something bad that that person has done.
Dan grassed on them to the local police. 丹向当地警方告发了他们。