The Collaborative International Dictionary of English v.0.48:

Incredulous \In*cred"u*lous\ (?; 135), a. [L. incredulus. See
   In- not, and Credulous.]
   [1913 Webster]
   1. Not credulous; indisposed to admit or accept that which is
      related as true, skeptical; unbelieving. --Bacon.
      [1913 Webster]

            A fantastical incredulous fool.       --Bp. Wilkins.
      [1913 Webster]

   2. Indicating, or caused by, disbelief or incredulity. "An
      incredulous smile." --Longfellow.
      [1913 Webster]

   3. Incredible; not easy to be believed. [R.] --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

incredulous
    adj 1: not disposed or willing to believe; unbelieving [ant:
           credulous]