The Collaborative International Dictionary of English v.0.48:

Exogamous \Ex*og"a*mous\, a. [Exo- + Gr. ? marriage.]
   Relating to exogamy; marrying outside of the limits of one's
   own tribe; -- opposed to endogenous.
   [1913 Webster]

WordNet (r) 3.0 (2006):

exogamous
    adj 1: characterized by or fit for fertilization by a flower
           that is not closely related [syn: exogamous,
           exogamic] [ant: autogamic, autogamous, endogamic,
           endogamous]
    2: pertaining to or characterized by the custom of marrying only
       outside the limits of a clan or tribe [syn: exogamous,
       exogamic] [ant: endogamic, endogamous]