mouth off

verb

1. informal,  to give an opinion or speak emotionally, often without much care or consideration

Idioms and Phrases

Complain or express one's opinions loudly and indiscreetly, as in She was always mouthing off about the other members . [ Slang ; 1960s]

Speak impudently, talk back, as in He got in trouble by mouthing off to his teacher . [ Slang ; 1960s]