rat on (one)
To inform an authority figure of one's bad or illegal behavior. I can't believe you ratted on me like that to mom and dad—I'm never telling you anything ever again! The criminal ratted on his accomplices to avoid jail time.
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.