run off at the mouth . run off at the mouth {v. phr.} To talk too much; be unable to stop talking. "Shut up, John," our father cried. "You are always running off at the mouth." Categories: add yours {v. phr.} Source: A Dictionary of American Idioms by A. Makkai, M.T. Boatner, J.E. Gates © 2024 » In RussianTo section: R Subscribe to: American IdiomsIdioms feedEnglish languageHome page and general feed Leave my opinion