The Collaborative International Dictionary of English v.0.48:

Impenetrability \Im*pen`e*tra*bil"i*ty\, n. [Cf. F.
   imp['e]n['e]trabilit['e].]
   1. Quality of being impenetrable.
      [1913 Webster]

   2. (Physics) That property in virtue of which two portions of
      matter can not at the same time occupy the same portion of
      space.
      [1913 Webster]

   3. Insusceptibility of intellectual or emotional impression;
      obtuseness; stupidity; coldness.
      [1913 Webster]

WordNet (r) 3.0 (2006):

impenetrability
    n 1: the quality of being impenetrable (by people or light or
         missiles etc.) [syn: impenetrability, imperviousness]
         [ant: penetrability, perviousness]
    2: incomprehensibility by virtue of being too dense to
       understand [syn: impenetrability, impenetrableness]