A closed book is something or somebody that is a mystery: something or somebody that you know nothing about, or don't understand.
Examples of use:
1. Our new neighbour is a closed book. Nobody knows who he is, or where he's from.
2. Global warming is a closed book to me.
3. I can't figure him out – he's a closed book.
4. Physics and chemistry were a closed book to me at school!
The expression 'it's all Greek to me' has a similar meaning.
Have you heard anybody use these expressions?
Can you make sentences with them?
Image © Brenda Clarke