1. Immune to attack; incapable of being tampered with
   "an impregnable fortress";
   - inviolable, secure, strong, unassailable, unattackable
    
2. Incapable of being overcome, challenged or refuted
   "an impregnable argument"; "impregnable self-confidence";
   - inexpugnable
    
3. Capable of being impregnated or fertilized
   "The female animals were impregnable during their fertile period";
   - conceptive [rare]

See also: fertile, invulnerable, unconquerable

Encyclopedia: Impregnable