impregnable in WordNet English dictionary

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