rat on someone
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)
in. to inform (on someone). Bill said he was going to rat on that punk.
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.