a closed book
Also found in: Wikipedia.
a closed bookBRITISH
If something or someone is a closed book to you, you know or understand very little about them. Economics were a closed book to him. He is such a closed book, that guy. Compare with an open book.
a closed booka thing of which you have no knowledge or understanding.
1944 Frank Clune The Red Heart The desert is an open book to the man of the Vast Open Spaces, but to the schoolmaster it was a closed book.