The Collaborative International Dictionary of English v.0.48:

Osage orange \O"sage or"ange\ (Bot.)
   An ornamental tree of the genus Maclura (Maclura
   aurantiaca), closely allied to the mulberry (Morus); also,
   its fruit. The tree was first found in the country of the
   Osage Indians, and bears a hard and inedible fruit of an
   orangelike appearance. See Bois d'arc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

osage orange
    n 1: small shrubby deciduous yellowwood tree of south central
         United States having spines, glossy dark green leaves and
         an inedible fruit that resembles an orange; its hard
         orange-colored wood used for bows by Native Americans;
         frequently planted as boundary hedge [syn: osage orange,
         bow wood, mock orange, Maclura pomifera]