The Collaborative International Dictionary of English v.0.48:

Osmious \Os"mi*ous\, a. (Chem.)
   Denoting those compounds of osmium in which the element has a
   valence relatively lower than in the osmic compounds; as,
   osmious chloride. [Written also osmous.]
   [1913 Webster]

   Osmious acid (Chem.), an acid derived from osmium,
      analogous to sulphurous acid, and forming unstable salts.
      It is a brown amorphous substance.
      [1913 Webster]