The Collaborative International Dictionary of English v.0.48:

Impregnable \Im*preg"na*ble\, a. [F. imprenable; pref. im- not +
   prenable pregnable, fr. prendre to take, L. prehendere. See
   Comprehend, Get to obtain.]
   Not to be stormed, or taken by assault; incapable of being
   subdued; able to resist attack; unconquerable; as, an
   impregnable fortress; impregnable virtue.
   [1913 Webster]

         The man's affection remains wholly unconcerned and
         impregnable.                             --South.
   -- Im*preg"na*ble*ness, n. -- Im*preg"na*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

impregnably
    adv 1: in an impregnable manner; "the sight of that bland,
           impregnably righteous face has been enough to make their
           blood run cold"