unthinkable

from WordNet (r) 3.0 (2006)

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

from The Collaborative International Dictionary of English v.0.48

Unthinkable \Unthinkable\
   See {thinkable}.

from 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