bulwark |ˈbo͝olˌwərk| noun 1 a defensive wall. • a person, institution, or principle that acts as a defense: the security forces are a bulwark against the breakdown of society. 2 (usu. bulwarks) an extension of a ship's sides above the level of the deck. ORIGIN late Middle English: from Middle Low German and Middle Dutch bolwerk; related to bole1 and work.