unsurmountable

from WordNet (r) 3.0 (2006)

unsurmountable
    adj 1: not capable of being surmounted or overcome;
           "insurmountable disadvantages" [syn: {insurmountable},
           {unsurmountable}] [ant: {surmountable}]
    2: incapable of being surmounted or climbed [syn:
       {unsurmountable}, {unclimbable}]

from The Collaborative International Dictionary of English v.0.48

Unsurmountable \Un`sur*mount"a*ble\, a.
   Insurmountable. --Locke.
   [1913 Webster]