The Collaborative International Dictionary
Impregnableness
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.
Wiktionary
impregnableness
n. The state of being impregnable; impregnability.