Source : Webster's Revised Unabridged Dictionary (1913)

Implausible \Im*plau"si*ble\, a. [Pref. im- not + plausible: cf.
   F. implausible.]
   Not plausible; not wearing the appearance of truth or
   credibility, and not likely to be believed. ``Implausible
   harangues.'' --Swift. -- {Im*plau"si*ble*ness}, n. --
   {Im*plau"si*bly}, adv.

Source : WordNet®

implausible
     adj 1: having a quality that provokes disbelief; "gave the teacher
            an implausible excuse" [ant: {plausible}]
     2: highly imaginative but unlikely; "a farfetched excuse"; "an
        implausible explanation" [syn: {farfetched}]