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.
Origin of closed book
First recorded in 1910–15
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
Related Words for closed bookriddle, conundrum, subtlety, question, enigma, secrecy, problem, thriller, mystery, dilemma, puzzle, quandary, complexity, teaser, puzzlement, mystification, perplexity, stickler, cliffhanger, twister
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