From Longman Dictionary of Contemporary Englisha closed book (to somebody)a closed book (to somebody)NOT KNOWa subject or problem that someone does not know about or understand Mathematics has always been a closed book to me. → closed
Examples from the Corpus
a closed book (to somebody)• I can not believe that it can be right that this late in the game Poetry is still a closed book.• The kitchenette is a closed book.• The Shoah will never be a closed book.• I tell myself it's a closed book, but my cover story becomes an old man's compensation.• But your own past can be a closed book, even at fourteen.• He is holding a closed book, signifying a mystery, possibly a stage in the alchemical process.• The highly organised St Stephen's Society programme which she now leads was at that time a closed book to her!• Linear preoccupation in the past remains a closed book to modern understanding. 