to know one's book
Preserved but no longer current.
Meaning · English
To know one's own interest; to know on which side one's bread is buttered. Also, to have made up one's mind.
Era: ca. 1898 (Victorian)
Sources & notes
Reproduced from Brewer’s Dictionary of Phrase and Fable (1898 edition, public domain; archive.org, retrieved 2026-07-31; OCR text, minor errors possible). Reflects the scholarship and attitudes of its era.
Common questions
What does “to know one's book” mean?
To know one's own interest; to know on which side one's bread is buttered. Also, to have made up one's mind.
Is “to know one's book” formal or informal?
No register label has been reviewed yet — treat it as neutral and check the usage notes above.