The Collaborative International Dictionary of English v.0.48:

Demosthenic \Dem`os*then"ic\, a. [L. Demosthenicus: cf. F.
   D['e]mosth['e]nique.]
   Pertaining to, or in the style of, Demosthenes, the Grecian
   orator.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Demosthenic
    adj 1: of or relating to Demosthenes or his oratory