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]

Moby Thesaurus II by Grady Ward, 1.0:

114 Moby Thesaurus words for "incredible":
   a bit thick, a bit thin, absurd, amazing, antic, appalling,
   astonishing, astounding, awe-inspiring, awesome, beguiling,
   bewildering, beyond belief, bizarre, cockamamie, conspicuous,
   crazy, doubtable, doubtful, dubious, dubitable, egregious,
   enigmatic, exceptional, extraordinary, extravagant, fabulous,
   fanciful, fantastic, fascinating, fictitious, flimsy, foolish,
   formidable, grotesque, hard of belief, hard to believe,
   hardly possible, high-flown, humongous, implausible, impossible,
   improbable, incomprehensible, inconceivable, laughable, ludicrous,
   marked, marvelous, miraculous, monstrous, more than doubtful,
   mythical, nonsensical, not deserving belief, notable, noteworthy,
   noticeable, of mark, open to doubt, open to suspicion, outlandish,
   outrageous, outre, outstanding, passing belief, passing strange,
   phenomenal, poppycockish, preposterous, problematic, prodigious,
   puzzling, questionable, rare, remarkable, ridiculous, sensational,
   signal, staggering belief, strange, striking, stupendous, superior,
   suspect, suspicious, tall, thick, thin, tremendous, unaccountable,
   unbelievable, uncommon, unconvincing, unearthly, unexpected,
   ungodly, unheard-of, unimaginable, unique, unlikely, unprecedented,
   unpromising, unreal, unrealistic, unsubstantial, untenable,
   unthinkable, unworthy of belief, weak, weird, wild, wonderful,
   wondrous