Webster’s Revised Unabridged Dictionary · 1913
Impregnable
Im·preg′na·ble (ĭm·prĕg′nȧ·b'l), 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.
Im·preg′na·ble, a. [See Impregnate.] (Biol.) Capable of being impregnated, as the egg of an animal, or the ovule of a plant.