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closed book

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noun
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.
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Origin of closed book

First recorded in 1910–15
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2021

How to use closed book in a sentence

British Dictionary definitions for closed book

closed book

noun
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
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