Entry 3 senses · 2 variants Webster, 1913 Osage · IPA /ˈoʊseɪdʒ/ prop. n. prop. n. 01 prop. n. A tributary of the Missouri River. 1. A tributary of the Missouri River. Syn. Osage River. 02 prop. n. A member of the Osages, a tribe of North American Indians formerly living in western Missouri. 1. A member of the Osages, a tribe of North American Indians formerly living in western Missouri. 2. The language of the Osages, a siouxan language.