From Medieval Latin 'firmarius' (one who pays fixed rent) — originally a tax collector, not an agricultural worker.
A person who owns or manages a farm; one who cultivates land or raises livestock for a living.
From Anglo-French 'fermer' (one who collects revenue, tenant), from 'ferme' (a lease, a fixed payment), from Medieval Latin 'firma' (fixed payment, rent), from Latin 'firmāre' (to fix, to settle, to confirm), from 'firmus' (firm, stable). The original 'farmer' was not someone who plowed fields — he was a tax collector. In medieval England, a 'fermor' was a person who leased the right to collect taxes