The Collaborative International Dictionary of English v.0.48:

Impervious \Im*per"vi*ous\, a. [L. impervius; pref. im- not +
   per through + via way. See Voyage.]
   Not pervious; not admitting of entrance or passage through;
   as, a substance impervious to water or air.
   [1913 Webster]

         This gulf impassable, impervious.        --Milton.
   [1913 Webster]

         The minds of these zealots were absolutely impervious.
                                                  --Macaulay.

   Syn: Impassable; pathless; impenetrable; imperviable;
        impermeable. -- Im*per"vi*ous*ly, adv. --
        Im*per"vi*ous*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

imperviousness
    n 1: the quality of being impenetrable (by people or light or
         missiles etc.) [syn: impenetrability, imperviousness]
         [ant: penetrability, perviousness]