a proof, originally one set from type in a galley, taken before the material has been made up into pages and usually printed as a single column of type with wide margins for marking corrections.
Origin
1885-90
British Dictionary definitions for galley proof
galley proof
noun
1.
a printer's proof, esp one taken on a long strip of paper from type in a galley, used to make corrections before the matter has been split into pages Often shortened to galley