Source : Webster's Revised Unabridged Dictionary (1913)

Arsenious \Ar*se"ni*ous\, a. [Cf. F. ars['e]nieux.]
   1. Pertaining to, consisting of, or containing, arsenic; as,
      arsenious powder or glass.

   2. (Chem.) Pertaining to, or derived from, arsenic, when
      having an equivalence next lower than the highest; as,
      arsenious acid.

Source : WordNet®

arsenious
     adj : relating to compounds in which arsenic is trivalent