The idiom "closed book" is believed to have originated in the 19th century. It is thought to have come from the literal meaning of a closed book, which is a book that is not open and cannot be read. The phrase "closed book" came to be used to describe someone or something that one knows very little or nothing about. It suggests that the person or thing in question is a mystery to the speaker.

What is the origin of the idiom "closed book" and when to use it?

英語での「closed book」の定義と意味

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