Source : Webster's Revised Unabridged Dictionary (1913)

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}.]

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