fall flat . fall flat {v.}, {informal} To be a failure; fail. The party fell flat because of the rain. His joke fell flat because no one understood it. Categories: add yours {informal} {v.} Source: A Dictionary of American Idioms by A. Makkai, M.T. Boatner, J.E. Gates © 2025 To section: F Subscribe to: American IdiomsIdioms feedEnglish languageHome page and general feed Add my comment