blow the gaff . blow the gaff {v. phr.} To open one's mouth to reveal a secret. When Al cheated on his wife, his younger brother blew the gaff on him. Categories: add yours {v. phr.} Source: A Dictionary of American Idioms by A. Makkai, M.T. Boatner, J.E. Gates © 2024 To section: B Subscribe to: American IdiomsIdioms feedEnglish languageHome page and general feed Leave my opinion