The Collaborative International Dictionary of English v.0.48:

Inexsuperable \In`ex*su"per*a*ble\, a. [L. inexsuperabilis;
   pref. in- not + exsuperabilis that may be surmounted. See
   In- not, Ex-, and Superable.]
   Not capable of being passed over; insuperable;
   insurmountable.
   [1913 Webster]