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]

Moby Thesaurus II by Grady Ward, 1.0:

76 Moby Thesaurus words for "sophistical":
   Machiavellian, Machiavellic, acute, apparently sound, arch, artful,
   astute, cagey, canny, casuistic, clever, colorable, crafty,
   cunning, cute, deceitful, deceptive, deep, deep-laid, designing,
   diplomatic, dishonest, disingenuous, empty, fallacious, feline,
   foxy, guileful, hollow, illusive, ingenious, insidious, insincere,
   inventive, jesuitic, knowing, mealymouthed, overrefined,
   oversubtle, pawky, philosophicojuristic, philosophicoreligious,
   philosophistic, plausible, politic, ready, resourceful, scheming,
   serpentine, sharp, shifty, shrewd, slick, slippery, sly, smooth,
   snaky, sneaky, sophistic, specious, stealthy, strategic, subtile,
   subtle, supple, tactical, tongue in cheek, trickish, tricksy,
   tricky, uncandid, unfrank, unserious, vulpine, wary, wily