Source : Webster's Revised Unabridged Dictionary (1913)

Insurmountable \In`sur*mount"a*ble\, a. [Pref. in- not +
   surmountable: cf. F. insurmountable.]
   Incapable of being passed over, surmounted, or overcome;
   insuperable; as, insurmountable difficulty or obstacle.
   --Locke.

         Hope thinks nothing difficult; despair tells us that
         difficulty is insurmountable.            --I. Watts.

   Syn: Insuperable; impassable; invincible.

Source : WordNet®

insurmountable
     adj 1: not capable of being surmounted or overcome; "insurmountable
            disadvantages" [syn: {unsurmountable}] [ant: {surmountable}]
     2: impossible to surmount [syn: {insuperable}]