The Collaborative International Dictionary of English v.0.48:

Thionic \Thi*on"ic\, a. [Gr. ? brimstone, sulphur.] (Chem.)
   Of or pertaining to sulphur; containing or resembling
   sulphur; specifically, designating certain of the thio
   compounds; as, the thionic acids. Cf. Dithionic,
   Trithionic, Tetrathionic, etc.
   [1913 Webster]