The Collaborative International Dictionary of English v.0.48:

Specious \Spe"cious\, a. [L. speciosusgood-looking, beautiful,
   specious, fr. species look, show, appearance; cf. F.
   sp['e]coeux. See Species.]
   1. Presenting a pleasing appearance; pleasing in form or
      look; showy.
      [1913 Webster]

            Some [serpents] specious and beautiful to the eye.
                                                  --Bp.
                                                  Richardson.
      [1913 Webster]

            The rest, far greater part,
            Will deem in outward rites and specious forms
            Religion satisfied.                   --Milton.
      [1913 Webster]

   2. Apparently right; superficially fair, just, or correct,
      but not so in reality; appearing well at first view;
      plausible; as, specious reasoning; a specious argument.
      [1913 Webster]

            Misled for a moment by the specious names of
            religion, liberty, and property.      --Macaulay.
      [1913 Webster]

            In consequence of their greater command of specious
            expression.                           --J. Morley.
      [1913 Webster]

   Syn: Plausible; showy; ostensible; colorable; feasible. See
        Plausible.
        [1913 Webster] -- Spe"xious*ly, adv. --
        Spe"cious*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

speciousness
    n 1: an appearance of truth that is false or deceptive; seeming
         plausibility; "the speciousness of his argument" [syn:
         speciousness, meretriciousness]

Moby Thesaurus II by Grady Ward, 1.0:

103 Moby Thesaurus words for "speciousness":
   acting, affectation, apparent character, apparent soundness,
   appearance, appearances, attitudinizing, bluff, bluffing,
   casuistry, cheating, circularity, color, coloring, deception,
   deceptiveness, delusion, disguise, disingenuousness, display,
   dissemblance, dissembling, dissimulation, distortion,
   equivocalness, equivocation, evasive reasoning, exteriors,
   external appearance, externals, facade, face, fakery, faking,
   fallaciousness, fallacy, false air, false front, false show,
   falsity, feigning, feint, four-flushing, fraud, front, gaudiness,
   gilt, gloss, humbug, humbuggery, imposture, insincerity, jesuitism,
   jesuitry, masquerade, mere externals, meretriciousness,
   misapplication, mystification, obfuscation, obscurantism, ostent,
   ostentation, outward appearance, outward show, oversubtlety,
   perversion, philosophism, plausibility, plausibleness, playacting,
   pose, posing, posture, pretense, pretension, pretext, public image,
   rationalization, representation, seeming, semblance, sham, show,
   simulacrum, simulation, sophism, sophistical reasoning,
   sophistication, sophistry, special pleading, speciosity,
   specious reasoning, spuriousness, subtlety, superficiality,
   surface appearance, surface show, vain show, varnish,
   vicious circle, vicious reasoning, window dressing