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):

impervious
    adj 1: not admitting of passage or capable of being affected; "a
           material impervious to water"; "someone impervious to
           argument" [syn: impervious, imperviable] [ant:
           pervious]

Moby Thesaurus II by Grady Ward, 1.0:

83 Moby Thesaurus words for "impervious":
   Philistine, airtight, ballproof, beyond reach, bombproof,
   bulletproof, burglarproof, callous, calloused, case-hardened,
   closed forever to, closed to, corrosionproof, dampproof,
   fire-resisting, fireproof, flameproof, flinty, foolproof, hard,
   hardened, hardhearted, hermetic, holeproof, impassable,
   impenetrable, impermeable, impersuadable, impersuasible,
   impervious to, inaccessible, indurated, inflexible, insensitive,
   inured, leakproof, lost to, noiseproof, nonpuncturable, obstinate,
   out of reach, pachydermatous, proof, proof against, punctureproof,
   resistant, rustproof, shatterproof, shellproof, soundproof,
   steeled against, steely, stony, thick-skinned, tight,
   un-come-at-able, unaccessible, unamenable, unapproachable,
   unattainable, unavailable, undiscoverable, ungettable,
   unimpressionable, uninfluenceable, unmovable, unobtainable,
   unpassable, unperforable, unpersuadable, unpierceable, unpliable,
   unprocurable, unreachable, unreceptive, unresponsive, unsecurable,
   unsuggestible, unsusceptible, unswayable, unyielding, watertight,
   weatherproof