"loose trousers of a strong material worn by cowboys, etc.," 1782, from over (adv.) + all. Specific sense "loose fitting canvas trousers with a bib and strap top" (originally worn by workmen over other clothes to protect them from wet, dirt, etc.) is attested by 1897. Compare French surtout "overcoat," literally "an over all," from sur- "over" + tout "all."
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