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Verb: mouth off
Usage: informal
  1. Talk in a noisy, excited, or declamatory manner
    "The teenager mouthed off to the teacher";
    - rant, jabber, spout, rabbit on [informal], rave

Derived forms: mouths off, mouthed off, mouthing off

See also: talk

Type of: mouth, speak, talk, utter, verbalise [Brit], verbalize