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]

The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

impregnable
    adj 1: immune to attack; incapable of being tampered with; "an
           impregnable fortress"; "fortifications that made the
           frontier inviolable"; "a secure telephone connection"
           [syn: impregnable, inviolable, secure, strong,
           unassailable, unattackable]
    2: capable of conceiving [syn: conceptive, impregnable]
    3: incapable of being overcome, challenged or refuted; "an
       impregnable argument"; "impregnable self-confidence" [syn:
       impregnable, inexpugnable]

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "impregnable":
   deathless, defended, guarded, immortal, impenetrable, imperishable,
   incontestable, incorruptible, indelible, indestructible,
   indomitable, ineffaceable, ineradicable, inexpugnable,
   inextinguishable, insuperable, invincible, inviolable,
   invulnerable, irresistible, mighty, nonperishable, overpowering,
   overwhelming, protected, quenchless, resistless, safe, safeguarded,
   secure, shielded, unassailable, unbeatable, unconquerable,
   undampable, undefeatable, undestroyable, undying, unquenchable,
   unsubduable, unsurmountable, unyielding