closed book, a

a closed book

Someone or something that is 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

closed book, a

A secret, mystery, or puzzle, as in I can't figure her out; she's a closed book to me. This term alludes to information one can't obtain or comprehend (because the book is closed). [Early 1900s]
See also: closed