The Collaborative International Dictionary of English v.0.48:

Insuperable \In*su"per*a*ble\, a. [L. insuperabilis: cf. OF.
   insuperable. See In- not, and Superable.]
   Incapable of being passed over or surmounted; insurmountable;
   as, insuperable difficulties.
   [1913 Webster]

         And middle natures, how they long to join,
         Yet never pass the insuperable line?     --Pope.
   [1913 Webster]

         The difficulty is enhanced, or is . . . insuperable.
                                                  --I. Taylor.

   Syn: Impassable; insurmountable; unconquerable. --
        In*su"per*a*ble*ness, n. -- In*su"per*a*bly, adv.
        [1913 Webster]

WordNet (r) 3.0 (2006):

insuperable
    adj 1: impossible to surmount [syn: insuperable,
           insurmountable]
    2: incapable of being surmounted or excelled; "insuperable
       odds"; "insuperable heroes" [syn: insuperable,
       unconquerable]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "insuperable":
   beyond one, impassable, impenetrable, impracticable, impractical,
   impregnable, incontestable, indomitable, inexpugnable, inoperable,
   insurmountable, invincible, inviolable, invulnerable, irresistible,
   overpowering, overwhelming, resistless, too much for, unachievable,
   unassailable, unattainable, unbeatable, uncompassable,
   unconquerable, undoable, uneffectible, unnegotiable, unperformable,
   unrealizable, unsubduable, unsurmountable, unsurpassable,
   unworkable, unyielding