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Adjective: impregnable  im'preg-nu-bul
  1. Immune to attack; incapable of being tampered with
    "an impregnable fortress";
    - inviolable, secure, strong, unassailable, unattackable
  2. Capable of conceiving
    - conceptive
  3. Incapable of being overcome, challenged or refuted
    "an impregnable argument"; "impregnable self-confidence";
    - inexpugnable

See also: fertile, invulnerable, unconquerable

Encyclopedia: Impregnable