From WordNet (r) 2.0 (August 2003) [wn]:
impassable
adj : impossible to pass [syn: unpassable] [ant: passable]
From The Collaborative International Dictionary of English v.0.48 [gcide]: 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]