Source : Webster's Revised Unabridged Dictionary (1913)

Unpassable \Un*pass"a*ble\, a.
   Impassable. --E. A. Freeman. -- {Un*pass"a*ble*ness}, n.
   Evelyn.

Source : WordNet®

unpassable
     adj : impossible to pass [syn: {impassable}] [ant: {passable}]