knock cold knock cold {v. phr.}, {informal} To render unconscious. The blow on the chin knocked Harry cold. Categories: add yours {informal} {v. phr.} Source: A Dictionary of American Idioms by A. Makkai, M.T. Boatner, J.E. Gates © 2024 To section: K Subscribe to: American IdiomsIdioms feedEnglish languageHome page and general feed Leave my opinion