mouth offvi phrasalphrasal verb, intransitive: Verb with adverb(s) or preposition(s), having special meaning and not taking direct object--for example, "make up" [=reconcile]: "After they fought, they made up."
slang (give opinion loudly)
毫无顾忌地高声谈论 háo wú gù jì de gāo shēng tán lùn
高谈阔论 gāo tán kuò lùn
The student got kicked out of class for mouthing off too much.