The Collaborative International Dictionary of English v.0.48:

Inamissible \In`a*mis"si*ble\, a. [L. inamissibilis: cf. F.
   inamissible.]
   Incapable of being lost. [R.] --Hammond. --
   In`a*mis"si*ble*ness, n. [R.]
   [1913 Webster]