The Collaborative International Dictionary of English v.0.48:

Octonary \Oc`to*na*ry\, a. [L. octonarius, fr. octoni eight
   each, fr. octo eight.]
   Of or pertaining to the number eight. --Dr. H. More.
   [1913 Webster]

WordNet (r) 3.0 (2006):

octonary
    n 1: the cardinal number that is the sum of seven and one [syn:
         eight, 8, VIII, eighter, eighter from Decatur,
         octad, ogdoad, octonary, octet]