1
general::
1. Complain or express one's opinions loudly and indiscreetly, as in She was always mouthing off about the other members. [Slang; 1960s] 2. Speak impudently, talk back, as in He got in trouble by mouthing off to his teacher. [Slang; 1960s]
American Heritage Idioms
2
general::
Phrase(s): mouth off
to speak out of turn; to backtalk. • If you mouth off, I will ground you for three weeks. • Don’t mouth off at me, buster!
McGrawhill's American Idioms And Phrasal Verbs