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):

sophistical
    adj 1: plausible but misleading [syn: sophistic,
           sophistical]