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"