Farmer

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late Middle English: from Old French fermier, from medieval Latin firmarius, firmator, from firma (see farm). farmer (sense 1) originally denoted a bailiff or steward who farmed land on the owner's behalf, or a tenant farmer.

wiktionary

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From Middle English fermour(“a steward, bailliff, collector of taxes”), from Old French fermier(“a farmer, a lessee, husbandman, bailliff”), from Medieval Latin firmarius(“one to whom land is rented, a collector of taxes, deputy”), from firma; equivalent to farm +‎ -er. Compare Old English feormere(“a purveyor of a guild, a supplier of food, a grocer, farmer”). More at farm.

etymonline

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farmer (n.)

late 14c., "one who collects taxes, etc.," from Anglo-French fermer, Old French fermier "lease-holder," from Medieval Latin firmarius, from firma "fixed payment" (see farm (n.)). In the agricultural sense, 1590s, replacing native churl and husbandman.