closed book, a

Idioms  

1. 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]

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Example Sentences

He gave his closed book a sullen look.

From In Connection with the De Willoughby Claim by Burnett, Frances Hodgson

It was a closed book, a balanced account.

From Big Timber A Story of the Northwest by Sinclair, Bertrand W.