The Collaborative International Dictionary of English v.0.48:

Phosphorous \Phos"phor*ous\ (f[o^]s"f[o^]r*[u^]s), a. [Cf. F.
   phosphoreux.] (Chem.)
   Of or pertaining to phosphorus; resembling or containing
   phosphorus; specifically, designating those compounds in
   which phosphorus has a lower valence as contrasted with
   phosphoric compounds; as, phosphorous acid, H3PO3.
   [1913 Webster]