The Collaborative International Dictionary of English v.0.48:

Incredibility \In*cred`i*bil"i*ty\, n. [L. incredibilitas: cf.
   F. incr['e]dibilit['e].]
   [1913 Webster]
   1. The quality or state of being incredible; incredibleness.
      --Dryden.
      [1913 Webster]

   2. That which is incredible. --Johnson.
      [1913 Webster]

WordNet (r) 3.0 (2006):

incredibility
    n 1: the quality of being incredible [syn: incredibility,
         incredibleness] [ant: believability, credibility,
         credibleness]