closed book
Proposed by a contributor, not yet supported by evidence.
Meaning · English
Completely unknown or mysterious
Literally: a book that is closed
Where it’s used:
Used
Widespread English
Examples
The company's financial dealings are a closed book to the public
AI-drafted example auto-approved by rule (illustrative, not a corpus attestation)
Sources & notes
Listed in Wiktionary’s “English idioms” category (retrieved 2026-07-29); definition pending editorial review. Definition drafted by AI (@cf/meta/llama-3.3-70b-instruct-fp8-fast) and auto-approved by rule on 2026-07-30; pending editor spot-check.
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Common questions
What does “closed book” mean?
Completely unknown or mysterious Literally: a book that is closed
Is “closed book” formal or informal?
No register label has been reviewed yet — treat it as neutral and check the usage notes above.
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