The Collaborative International Dictionary of English v.0.48:

Incredible \In*cred"i*ble\, a. [L. incredibilis: cf. OF.
   incredible. See In- not, and Credible.]
   Not credible; surpassing belief; too extraordinary and
   improbable to admit of belief; unlikely; marvelous; fabulous.
   [1913 Webster]

         Why should it be thought a thing incredible with you,
         that God should raise the dead?          --Acts xxvi.
                                                  8.
   [1913 Webster]

WordNet (r) 3.0 (2006):

incredible
    adj 1: beyond belief or understanding; "at incredible speed";
           "the book's plot is simply incredible" [syn:
           incredible, unbelievable] [ant: believable,
           credible]