be a closed book

be a closed book

To be difficult to know or understand. She's such a private person that I don't know much about her. I guess she's a closed book. I'm a scientist, so art will always be a closed book to me.
See also: book, closed

be a closed book

to be something that you know or understand nothing about (usually + to ) I'm afraid physics will always be a closed book to me.
See also: book, closed