Meaning of the word "closed book" in English

What does "closed book" mean in English? Explore the meaning, pronunciation, and specific usage of this word with Lingoland.

closed book

US /kloʊzd bʊk/
UK /kləʊzd bʊk/
"closed book" picture

Idiom

a subject or person about which one knows nothing or very little

Example:
Quantum physics is a closed book to me.
His past is a closed book; he never talks about it.