A closed book is an expression used when you accept something that has completely ended.
Something you don’t understand or know anything about.
A secret, mystery, or puzzle.
1. Today’s meeting is about terminating our project, it’s now a closed book.
2. I can’t figure her out, she’s a closed book to me.
3. Matthew is a closed book which makes it hard for me, as a teacher, to reach out to him.
4. I’m afraid mathematics will always be a closed book to me.
abstruseness, brainteaser, braintwister, charade, chiller, conundrum, crux, difficulty, mindboggler, perplexity, poser, problem, puzzle, puzzlement, question, secrecy, stickler, stumper, subtlety, teaser