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]