Mouth off Definition 1 of 2Pro Tip 1/3Informal Usage Used in casual settings among friends, but can be disrespectful in formal situations. He had no right to mouth off at the party like that. Pro Tip 2/3Negative Emotion Often shows the speaker is annoyed or has strong negative feelings. Stop mouthing off about the new policy; nobody likes it. Pro Tip 3/3Avoid in Professionalism Not recommended in workplaces or professional environments as it's seen as unprofessional. Mouthing off to the boss could cost you your job.