closed book

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.

1910–15 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
closed book
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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