The Collaborative International Dictionary of English v.0.48:

Implausibility \Im*plau`si*bil"i*ty\, n.
   Lack of plausibility; the quality of being implausible.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "implausibility":
   bare possibility, credibility gap, doubtfulness, dubiousness,
   faint likelihood, improbability, inconceivability, incredibility,
   little expectation, poor possibility, poor prospect,
   questionableness, small chance, unbelievability, unbelievableness,
   unlikelihood, unreliability, untenableness