The Collaborative International Dictionary of English v.0.48:

Theriac \The"ri*ac\, Theriaca \The*ri"a*ca\, n. [L. theriaca an
   antidote against the bite of serpents, Gr. ?: cf. F.
   th['e]riaque. See Treacle.]
   1. (Old Med.) An ancient composition esteemed efficacious
      against the effects of poison; especially, a certain
      compound of sixty-four drugs, prepared, pulverized, and
      reduced by means of honey to an electuary; -- called also
      theriaca Andromachi, and Venice treacle.
      [1913 Webster]

   2. Treacle; molasses. --British Pharm.
      [1913 Webster] Theriac