The Collaborative International Dictionary of English v.0.48:

Trisyllabic \Tris`yl*lab"ic\, Trisyllabical \Tris`yl*lab"ic*al\,
   a. [L. trisyllabus, Gr. ?; ? (see Tri-) + ? a syllable: cf.
   F. trissyllabique.]
   Of or pertaining to a trisyllable; consisting of three
   syllables; as, "syllable" is a trisyllabic word. --
   Tris`yllab"ic*al*ly, adv.
   [1913 Webster]