closed book
Plural
closed books
1
someone or something one knows very little or nothing about
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.
- The workings of the stock market are a closed book to me – I don't understand how it works.
- The new technology is a closed book to most people – they're not familiar with it yet.
- My new neighbor is a closed book to me – I don't know anything about them.
- The mysterious stranger was a closed book to everyone in the village – no one knew who he was or where he came from.
- The reclusive scientist was a closed book to his colleagues – he rarely interacted with them and never shared his work.