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.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
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]
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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.
The American Heritage® Dictionary of Phrasal Verbs. Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.