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.See also: on, rat

rat (on someone)

in. to inform (on someone). Bill said he was going to rat on that punk. See also: on, rat, someone