Definition from Wiktionary, the free dictionary
- (intransitive, idiomatic) To complain or otherwise express oneself in a loud, immoderate manner.
- (intransitive, idiomatic) To talk impudently, especially to one's superior.
2008, Herbert, “Play It Again, Brian”, in Family Guy:
- And don't you mouth off to me, or I'm gonna slap you right in your penis.