master key master key {n. phr.} A key that opens a set of different locks. The building janitor has a master key to all of the apartments in this building. Categories: add yours {n. phr.} Source: A Dictionary of American Idioms by A. Makkai, M.T. Boatner, J.E. Gates © 2024 » In RussianTo section: M Subscribe to: American IdiomsIdioms feedEnglish languageHome page and general feed Comment on this entry