Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

Source : WordNet®

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