The Collaborative International Dictionary of English v.0.48:

Inconceivable \In`con*ceiv"a*ble\, a. [Pref. in- not +
   conceivable: cf. F. inconcevable.]
   Not conceivable; incapable of being conceived by the mind;
   not explicable by the human intellect, or by any known
   principles or agencies; incomprehensible; as, it is
   inconceivable to us how the will acts in producing muscular
   motion.
   [1913 Webster]

         It is inconceivable to me that a spiritual substance
         should represent an extended figure.     --Locke.
   -- In`con*ceiv"a*ble*ness, n. -- In`con*ceiv"a*bly, adv.
   [1913 Webster]

         The inconceivableness of a quality existing without any
         subject to possess it.                   --A. Tucker.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

82 Moby Thesaurus words for "inconceivable":
   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, incredible,
   logically impossible, marvelous, mind-boggling, miraculous,
   not deserving belief, not possible, open to doubt,
   open to suspicion, outlandish, overwhelming, oxymoronic,
   paradoxical, passing belief, passing strange, phenomenal,
   preposterous, problematic, prodigious, prohibited, puzzling,
   questionable, rare, remarkable, ridiculous, ruled-out,
   self-contradictory, sensational, staggering, staggering belief,
   strange, striking, stupendous, suspect, suspicious, tall, thick,
   thin, unaccountable, unbelievable, unconvincing, unearthly,
   unexpected, ungodly, unheard-of, unimaginable, unique, unknowable,
   unprecedented, unthinkable, unworthy of belief, weak, wonderful,
   wondrous