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
Farlex Dictionary of Idioms.

rat (on someone)

in. to inform (on someone). Bill said he was going to rat on that punk.
See also: on, rat, someone
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions
See also:
  • rat (one) out
  • rat on
  • rat on (one)
  • rat out
  • ratted
  • going to tell
  • tattle
  • going to tattle
  • grass on (one)
  • grasse