Source : Webster's Revised Unabridged Dictionary (1913)

Unsurmountable \Un`sur*mount"a*ble\, a.
   Insurmountable. --Locke.

Source : WordNet®

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