rat on someone

(idiom) to tell someone in authority about the wrong or illegal actions of another.

Example Sentences:

My brother came home drunk so I ratted on him to my mother.

The ex-mobster ratted on his former gang mates when the police caught him.

Similar idioms:

1. rat someone out
2. rat
3. raise eyebrows
4. bark up the wrong tree
5. get up on the wrong side of the bed