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