The Collaborative International Dictionary of English v.0.48:

Hydrosulphurous \Hy`dro*sul"phur*ous\, a. (Chem.)
   Pertaining to, or designating, an acid obtained by the
   reduction of sulphurous acid. See Hyposulphurous acid,
   under Hyposulphurous.
   [1913 Webster]