seize on . seize on or upon {v. phr.} To latch onto. Whenever Herb is in a romantic mood, Irene seizes on it and starts talking about marriage, which is not what Herb had in mind. Categories: add yours mood {v. phr.} Source: A Dictionary of American Idioms by A. Makkai, M.T. Boatner, J.E. Gates © 2024 To section: S Subscribe to: American IdiomsIdioms feedEnglish languageHome page and general feed Leave my opinion