The Collaborative International Dictionary of English v.0.48:

Unbelievable \Unbelievable\
   See believable.

WordNet (r) 3.0 (2006):

unbelievable
    adj 1: beyond belief or understanding; "at incredible speed";
           "the book's plot is simply incredible" [syn:
           incredible, unbelievable] [ant: believable,
           credible]
    2: having a probability too low to inspire belief [syn:
       improbable, unbelievable, unconvincing, unlikely]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "unbelievable":
   a bit thick, a bit thin, absurd, antic, beyond belief, doubtable,
   doubtful, dubious, dubitable, fanciful, fantastic, flimsy,
   hard of belief, hard to believe, implausible, impossible,
   improbable, incomprehensible, inconceivable, incredible,
   mind-boggling, not deserving belief, open to doubt,
   open to suspicion, passing belief, preposterous, problematic,
   questionable, ridiculous, staggering belief, suspect, suspicious,
   tall, thick, thin, unaccountable, unconvincing, unearthly,
   unexpected, ungodly, unimaginable, unsubstantial, unthinkable,
   unworthy of belief, weak