E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A proof is a rough impression of a manuscript set up in type, or of a drawing engraved, for the authors correction. The proof with many faults is a foul proof, but the pull, after the errors are corrected, is termed a clean proof. These impressions are called proofs because they must be approved of by author and reader before they are finally printed.