Source : Webster's Revised Unabridged Dictionary (1913)

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.

         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.

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

Source : WordNet®

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