The Collaborative International Dictionary of English v.0.48:

Impassible \Im*pas"si*ble\, a. [L. impassibilis; pref. im- not +
   passibilis passable: cf. F. impassible. See Passible.]
   Incapable of suffering; inaccessible to harm or pain; not to
   be touched or moved to passion or sympathy; unfeeling, or not
   showing feeling; without sensation. "Impassible to the
   critic." --Sir W. Scott.
   [1913 Webster]

         Secure of death, I should contemn thy dart
         Though naked, and impassible depart.     --Dryden.
   [1913 Webster]