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

Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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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
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Example sentences
The exam should be closed book and closed notes, but seen beforehand.
She manages to be at once a closed book and a constant noisemaker.
If you do not earn the required credits, a closed book recertification examination is available as an alternative.
The examinations are closed book, so no reference materials, papers or study materials are allowed at the test center.
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