ostcommon nounproper nounEtymologyexpandfrom Old French ost which was pronounced /o/ (like tost, "soon"). The modern pronunciation is a reformation based on the spelling.Definitions1. Or Army in the Middle Ages.literaryexpandLa bannière qui conduisait jadis les cent lances du baron Palamède de Sigognac contre l'ost des Sarrasins.2. Troops provided by a vassal to his suzerain.flags.vieilliexpandPluralosts