closed book, a

a closed book

Someone or something that is difficult to know or understand. She's such a private person that I don't actually know very much about her. I guess I married a closed book. I'm a scientist through and through, so art has always been a closed book to me.
See also: book, closed
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

closed book, a

A secret, mystery, or puzzle, as in I can't figure her out; she's a closed book to me. This term alludes to information one can't obtain or comprehend (because the book is closed). [Early 1900s]
See also: closed
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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