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]