What do a.m. and p.m. stand for?

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.
Origin of closed book
1910-15 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2015.
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Examples from the web for closed book
  • 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.
British Dictionary definitions for closed book

closed book

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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