a closed book

• 未揭开的秘密,高深莫测的谜

a closed book的用法和样例：

例句

1. Fishing is a closed book to me.
   我对钓鱼是一窍不通。
2. Nuclear physics is a closed book to me.
   核物理学是我完全不懂的一门学科。
3. Nuclear physics is a closed book to most of us.
   我们大多数人对原子核物理学一窍不通。