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.
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            In consequence of their greater command of specious
            expression.                           --J. Morley.
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   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):

specious
    adj 1: plausible but false; "a specious claim"; "spurious
           inferences" [syn: specious, spurious]
    2: based on pretense; deceptively pleasing; "the gilded and
       perfumed but inwardly rotten nobility"; "meretricious
       praise"; "a meretricious argument" [syn: gilded,
       meretricious, specious]

Moby Thesaurus II by Grady Ward, 1.0:

85 Moby Thesaurus words for "specious":
   Barmecidal, Barmecide, airy, alleged, apparent, apparently sound,
   apparitional, autistic, avowed, beguiling, casuistic, chimeric,
   claimed, colorable, colored, conceivable, deceptive, delusional,
   delusionary, delusive, delusory, dereistic, disingenuous,
   dreamlike, dreamy, empty, erroneous, fallacious, false, fantastic,
   gilded, hollow, hypocritical, idle, illogical, illusional,
   illusionary, illusive, illusory, imaginary, in name only,
   inaccurate, incorrect, insincere, jesuitic, likely, meretricious,
   misleading, nugatory, ostensible, overrefined, oversubtle,
   phantasmagoric, phantasmal, phantom, philosophistic, plausible,
   possible, presumable, presumed, pretended, pretexted, professed,
   purported, seeming, self-deceptive, self-deluding, so-called,
   sophistic, sophistical, spectral, spurious, superficial, supposed,
   supposititious, tinsel, unactual, unfounded, unreal, unsound,
   unsubstantial, untrue, vain, visionary, wrong