impregnable

Adjective

• incapable of being overcome, challenged or refuted
  • an impregnable argument
    
  • impregnable self-confidence
    

• capable of conceiving

• immune to attack; incapable of being tampered with
  • an impregnable fortress
    
  • fortifications that made the frontier inviolable
    
  • a secure telephone connection
    

Related words

• Synonyms: 
  inexpugnable , conceptive , inviolable , secure , strong , unassailable , unattackable