The Collaborative International Dictionary of English v.0.48:

Impassable \Im*pass"a*ble\, a. [Cf. Unpassable.]
   Incapable of being passed; not admitting a passage; as, an
   impassable road, mountain, or gulf. --Milton. --
   Im*pass"a*ble*ness, n. -- Im*pass"a*bly, adv.
   [1913 Webster]