Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

         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.

Source : WordNet®

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