The Collaborative International Dictionary of English v.0.48:

Inspissate \In*spis"sate\, v. t. [imp. & p. p. Inspissated; p.
   pr. & vb. n. Inspissating.] [L. inspissatus, p. p. of
   inspissare to thicken; pref. in- + spissare to thicken, fr.
   spissus thick.]
   To thicken or bring to greater consistence, as fluids by
   evaporation.
   [1913 Webster]