The Collaborative International Dictionary of English v.0.48:

Oxonic \Ox*on"ic\, a. [Prob. glyoxalic + carbonic.] (Chem.)
   Pertaining to, or designating, a complex nitrogenous acid
   (C4H5N3O4) not known in the free state, but obtained, in
   combination with its salts, by a slow oxidation of uric acid,
   to which it is related.
   [1913 Webster]