closed book 一无所知，完全不懂，完全不了解的人

【例句】
Nuclear physics is a closed book to most of us.
我们大多数人对原子核物理学一无所知。

Frank Spriggs was a very able man but something of a closed book.
弗兰克·斯普里格斯是一个很有才干的人，但是有些让人捉摸不透。