clam up clam up {v.}, {slang} To refuse to say anything more; stop talking. The suspect clammed up, and the police could get no more information out of him. Categories: add yours {slang} {v.} Source: A Dictionary of American Idioms by A. Makkai, M.T. Boatner, J.E. Gates © 2024 To section: C Subscribe to: American IdiomsIdioms feedEnglish languageHome page and general feed Post my comment