The Collaborative International Dictionary of English v.0.48:

Sulphacid \Sulph*ac"id\, n. [Sulpho- + acid.] (Chem.)
   An acid in which, to a greater or less extent, sulphur plays
   a part analogous to that of oxygen in an oxyacid; thus,
   thiosulphuric and sulpharsenic acids are sulphacids; --
   called also sulphoacid. See the Note under Acid, n., 2.
   [1913 Webster]