The Collaborative International Dictionary of English v.0.48:

Unimaginable \Unimaginable\
   See imaginable.

WordNet (r) 3.0 (2006):

unimaginable
    adj 1: totally unlikely [syn: impossible, inconceivable,
           out of the question, unimaginable]

Moby Thesaurus II by Grady Ward, 1.0:

83 Moby Thesaurus words for "unimaginable":
   a bit thick, a bit thin, absurd, antic, barred, beguiling,
   bewildering, beyond belief, closed-out, contrary to reason,
   doubtable, doubtful, dubious, dubitable, enigmatic, exceptional,
   excluded, extraordinary, fabulous, fanciful, fantastic,
   fascinating, hard of belief, hard to believe, hopeless,
   implausible, impossible, incomprehensible, inconceivable,
   incredible, logically impossible, marvelous, miraculous,
   not deserving belief, not possible, open to doubt,
   open to suspicion, outlandish, oxymoronic, paradoxical,
   passing belief, passing strange, phenomenal, preposterous,
   problematic, prodigious, prohibited, puzzling, questionable, rare,
   remarkable, ridiculous, ruled-out, self-contradictory, sensational,
   singular, staggering belief, strange, striking, stupendous,
   suspect, suspicious, tall, thick, thin, unaccountable,
   unbelievable, uncommon, unconvincing, unearthly, unexpected,
   ungodly, unheard-of, unique, unknowable, unordinary, unprecedented,
   unthinkable, unusual, unwonted, unworthy of belief, wonderful,
   wondrous