The Collaborative International Dictionary of English v.0.48:
Unpassable \Un*pass"a*ble\, a. Impassable. --E. A. Freeman. -- Un*pass"a*ble*ness, n. --Evelyn. [1913 Webster]WordNet (r) 3.0 (2006):
unpassable adj 1: incapable of being passed [syn: impassable, unpassable] [ant: passable]