something that is not known or cannot be understood; a mystery or puzzle: Abstract art is a closed book as far as I'm concerned.
12 Quotes Book Lovers Can Relate ToIf you're a word lover you're probably a book lover too. These authors summarize our love for books perfectly in these quotes.
Origin of closed book
First recorded in 1910–15
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for closed book
something deemed unknown or incapable of being understood
a matter that has been finally concluded and admits of no further consideration
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012