adj .
incapable of being penetrated ; impervious ; incomprehensible 꿰뚫을 수 없는 ; 통과시키지 않는 ; 이해할 수 없는

• The fortress on the top of the hill was impenetrable to the poorly armed soldiers ; although they tried for days, they were unable to break through its thick stone walls.
  
• For obvious reasons, nights in the Middle Ages hoped that their armor would be impenetrable.
• This essay is utterly impenetrable. There isn't one word in it that makes sense to me.
  
• I was unable to guess what Bob was thinking ; as usual, his expression was impenetrable.
  밥이 무엇을 생각하고 있는지 짐작조차 할 수 없었다 ; 여느 때처럼 그의 표정은 이해하기 힘들었다.