The Collaborative International Dictionary of English v.0.48:

Thiosulphuric \Thi`o*sul*phur"ic\, a. [Thio- + sulphuric.]
   (Chem.)
   Of, pertaining to, or designating, an unstable acid,
   H2S2O3, analogous to sulphuric acid, and formerly called
   hyposulphurous acid.
   [1913 Webster]