The Collaborative International Dictionary of English v.0.48:

Sesquisulphide \Ses`qui*sul"phide\, n. [Sesqui- + sulphide.]
   (Chem.)
   A sulphide, analogous to a sesquioxide, containing three
   atoms of sulphur to two of the other ingredient; -- formerly
   called also sesquisulphuret; as, orpiment, As2S3 is
   arsenic sesquisulphide.
   [1913 Webster]