mouth off

From mouth + off.

mouth off (third-person singular simple present mouths off, present participle mouthing off, simple past and past participle mouthed off)

1. (intransitive, slang) To complain or otherwise express oneself in a loud, immoderate manner.
2. (transitive, intransitive, slang) 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.

• shoot one's mouth off

• loudmouth