impassable

from WordNet (r) 3.0 (2006)

impassable
    adj 1: incapable of being passed [syn: {impassable},
           {unpassable}] [ant: {passable}]

from 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]