The Collaborative International Dictionary of English v.0.48:

Chlorous \Chlo"rous\, a. [See Chlorine.]
   1. Of, pertaining to, or derived from, chlorine; -- said of
      those compounds of chlorine in which this element has a
      valence of three, the next lower than in chloric
      compounds; as, chlorous acid, HClO2.
      [1913 Webster]

   2. (Chem. Physics) Pertaining to, or resembling, the
      electro-negative character of chlorine; hence,
      electro-negative; -- opposed to basylous or zincous.
      [Obs.]
      [1913 Webster]