What does "mouth off" mean?

To complain or otherwise express oneself in a loud, immoderate manner.

What is a synonym for "mouth off"?

Common synonyms include: gobbed, jabber, rabbit on, rant, rave, shoot one's mouth off.

verb, slang

verb, slang ·Moderate ·College level

Verb

1
To complain or otherwise express oneself in a loud, immoderate manner. intransitive, slang
2
talk in a noisy, excited, or declamatory manner wordnet
3
To talk impudently, especially to one's superior. ambitransitive, slang
"And don't you mouth off to me, or I'm gonna slap you right in your penis."

"And don't you mouth off to me, or I'm gonna slap you right in your penis."

From mouth + off.