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.

Impregnable \Im*preg"na*ble\, a. [See Impregnate.] (Biol.)
   Capable of being impregnated, as the egg of an animal, or the
   ovule of a plant.

Source : WordNet®

impregnable
     adj 1: able to withstand attack; "an impregnable fortress";
            "fortifications that made the frontier inviolable"
            [syn: {inviolable}, {secure}, {strong}, {unassailable},
             {unattackable}]
     2: impossible to take by storm [syn: {inexpugnable}]