impenetrable in WordNet English dictionary

1. permitting little if any light to pass through because of denseness of matter
   "dense smoke"; "heavy fog"; "impenetrable gloom"
   dense  heavy  impenetrable 
   
2. not admitting of penetration or passage into or through
   "an impenetrable fortress"; "impenetrable rain forests"
   impenetrable 
   
3. impossible to understand
   "impenetrable jargon"
   impenetrable