The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

implausibleness
    n 1: the quality of provoking disbelief [syn: implausibility,
         implausibleness] [ant: plausibility, plausibleness]