to report someone's bad behavior; to tattle on someone. (Slang.)
John ratted on me, and I got in trouble.If he rats on me, I'll hit him!
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)
in. to inform (on someone). Bill said he was going to rat on that punk.