Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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