on board on board (1) {prep.} On (a ship). Joan was not on board the ship when it sailed. Categories: add yours {prep.} Source: A Dictionary of American Idioms by A. Makkai, M.T. Boatner, J.E. Gates © 2024 To section: O Subscribe to: American IdiomsIdioms feedEnglish languageHome page and general feed Comment on this entry