The Collaborative International Dictionary of English v.0.48:

Unthinkable \Unthinkable\
   See thinkable.

WordNet (r) 3.0 (2006):

unthinkable
    adj 1: incapable of being conceived or considered [ant:
           thinkable]

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "unthinkable":
   a bit thick, a bit thin, absurd, barred, beyond belief, closed-out,
   contrary to reason, doubtable, doubtful, dubious, dubitable,
   excluded, extraordinary, hard of belief, hard to believe, hopeless,
   illogical, implausible, impossible, improbable, incomprehensible,
   inconceivable, incredible, laughable, logically impossible,
   ludicrous, mind-blowing, mind-boggling, not deserving belief,
   not possible, open to doubt, open to suspicion, oxymoronic,
   paradoxical, passing belief, preposterous, problematic, prohibited,
   questionable, rare, ridiculous, ruled-out, self-contradictory,
   singular, staggering belief, suspect, suspicious, tall, thick,
   thin, unacceptable, unbelievable, uncommon, unconvincing,
   unearthly, ungodly, unimaginable, unique, unlikely, unordinary,
   unusual, unwonted, unworthy of belief