inexsuperable

from 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]