The Collaborative International Dictionary of English v.0.48:

Plausible \Plau"si*ble\, a. [L. plausibilis praiseworthy, from
   plaudere, plausum, to applaud, clap the hands, strike, beat.]
   1. Worthy of being applauded; praiseworthy; commendable;
      ready. [Obs.] --Bp. Hacket.
      [1913 Webster]

   2. Obtaining approbation; specifically pleasing; apparently
      right; specious; as, a plausible pretext; plausible
      manners; a plausible delusion. "Plausible and popular
      arguments." --Clarendon.
      [1913 Webster]

   3. Using specious arguments or discourse; as, a plausible
      speaker.
      [1913 Webster]
      [1913 Webster]

   Syn: Plausible, Specious.

   Usage: Plausible denotes that which seems reasonable, yet
          leaves distrust in the judgment. Specious describes
          that which presents a fair appearance to the view and
          yet covers something false. Specious refers more
          definitely to the act or purpose of false
          representation; plausible has more reference to the
          effect on the beholder or hearer. An argument may by
          specious when it is not plausible because its
          sophistry is so easily discovered.
          [1913 Webster]

WordNet (r) 3.0 (2006):

plausible
    adj 1: apparently reasonable and valid, and truthful; "a
           plausible excuse" [ant: implausible]
    2: given to or characterized by presenting specious arguments;
       "a plausible liar"

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "plausible":
   acceptable, admissible, apparent, apparently sound, believable,
   casuistic, cogent, cogitable, colorable, colored, conceivable,
   conceivably possible, contingent, credible, creditable, deceitful,
   deceptive, disingenuous, empty, fallacious, fiduciary, gilded,
   hollow, humanly possible, illusive, imaginable, insincere,
   jesuitic, just, justifiable, legitimate, likely, logical,
   meretricious, misleading, ostensible, overrefined, oversubtle,
   philosophistic, possible, potential, presentable, probable,
   rational, reasonable, reliable, sane, seeming, sensible, smooth,
   sophistic, sophistical, sound, specious, tenable, thinkable,
   tinsel, trustworthy, trusty, unexceptionable, unimpeachable,
   unquestionable, well-argued, well-founded, well-grounded,
   wholesome, worthy of faith