closed-book

closed book

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.

Origin:
1910–15

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Based on the Random House Dictionary, © Random House, Inc. 2014.
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Collins
World English Dictionary
closed book
 
n
1.  something deemed unknown or incapable of being understood
2.  a matter that has been finally concluded and admits of no further consideration

Collins English Dictionary - Complete & Unabridged 10th Edition
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