rat on someone
Inf. to report someone's bad behavior to someone in authority; to tattle on someone. John ratted on me, and I got in trouble. If he rats on me, I'll hit him!
Betray a comrade by giving information, as in He ratted on his best friend to the police. [Slang; early 1900s]
To disclose incriminating information about someone; betray someone by giving incriminating information: The hoodlum ratted on the rest of the gangsters. It was mean of you to rat on your friends.