The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

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

   Syn: Insuperable; impassable; invincible.
        [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "insurmountable":
   beyond one, impassable, impracticable, impractical, indomitable,
   inoperable, insuperable, invincible, too much for, unachievable,
   unattainable, uncompassable, unconquerable, undoable, uneffectible,
   unnegotiable, unperformable, unrealizable, unsurmountable,
   unworkable