The Collaborative International Dictionary of English v.0.48:

Octosyllabic \Oc`to*syl*lab"ic\, Octosyllabical
\Oc`to*syl*lab"ic*al\, a. [L. octosyllabus. See Octo-, and
   Syllable.]
   Consisting of or containing eight syllables.
   [1913 Webster]