The Collaborative International Dictionary of English v.0.48:

Sophistic \So*phis"tic\, Sophistical \So*phis"tic*al\, a. [L.
   sophisticus, Gr. ?: cf. F. sophistique.]
   Of or pertaining to a sophist; embodying sophistry;
   fallaciously subtile; not sound.
   [1913 Webster]

         His argument . . . is altogether sophistical.
                                                  --Macaulay.
   [1913 Webster] -- So*phis"tic*al*ly, adv. --
   So*phis"tic*al*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sophistic
    adj 1: of or pertaining to sophists
    2: plausible but misleading [syn: sophistic, sophistical]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "sophistic":
   baseless, bogus, captious, casuistical, deceptive, fallacious,
   false, groundless, hypocritical, invalid, irrational, mad,
   misleading, nonrational, reasonless, sham, sophistical, specious,
   unreasonable, unsound, untenable