Origins
The English word 'farmer' has an etymology that surprises nearly everyone who encounters it. It has nothing to do with soil, seeds, or harvests. Instead, it derives from Anglo-French 'fermer,' meaning 'one who collects revenue' or 'one who leases,' from 'ferme' (a lease, a fixed payment), which traces back through Medieval Latin 'firma' (a fixed payment or rent) to Latin 'firmāre' (to make firm, to fix, to settle) and ultimately to the adjective 'firmus' (firm, stable, strong). The farmer, etymologically, is 'the one who makes a fixed deal.'
The connection between tax collection and agriculture lies in the medieval system of 'farming out' revenues. In medieval England and France, the Crown or a feudal lord would lease ('farm out') the right to collect taxes, tolls, or other revenues in a particular area to a private individual, the 'fermor' or 'farmer.' This person paid a fixed sum upfront (the 'farm,' from 'firma') and then collected whatever he could, keeping the surplus as profit. This practice, known as 'tax farming,' was the primary meaning of 'farmer' well into the 15th century. Chaucer's 'fermour' is a revenue collector, not a plowman.
The semantic shift from tax collector to agriculturalist happened through a specific intermediate step: the leasing of agricultural land. When a landlord leased his estate to a tenant for a fixed annual rent (a 'farm'), that tenant was called a 'farmer' — not because he grew crops, but because he paid a 'firma.' Over time, since most such tenants were in fact cultivating the land they leased, the word 'farmer' became associated with the agricultural activity rather than the financial arrangement. By the 16th century, 'farmer' primarily meant a person who works the land, and the tax-collection sense had faded. The phrase 'tax farming,' still used in economics and history, is the last living trace of the original meaning.
Latin Roots
The Latin root 'firmus' (firm, stable) has generated an enormous family of English words through both direct borrowing and French transmission. 'Firm' (adjective and noun — a business firm is a 'fixed' or 'confirmed' entity), 'confirm' (to make firm), 'affirm' (to assert firmly), 'infirm' (not firm, weak), 'infirmary' (a place for the weak), and 'firmament' (the sky conceived as a solid, firm dome) all derive from the same root. The conceptual thread connecting these is the idea of fixedness, stability, and settlement — the farmer's 'farm' was originally a 'fixed payment,' just as a 'firm' is a 'fixed, confirmed' business arrangement.
Before 'farmer' assumed its agricultural meaning, English had other words for the person who worked the land. Old English used 'eorþtilia' (earth-tiller), and the Middle English word 'husbandman' (from Old Norse 'húsbóndi,' householder) was the standard term for a cultivator through the medieval period. 'Plowman' and 'yeoman' also served. The triumph of 'farmer' over these competitors is a linguistic accident of social history — the leasehold system became so prevalent that the lessee's financial title displaced all the older, more descriptive terms.
The word 'farm' itself underwent the same semantic journey. In Middle English, a 'ferme' was a fixed payment, a lease, or a rented property. 'To farm' meant 'to lease' or 'to rent out.' Only gradually did 'farm' come to mean the land itself, and then specifically agricultural land. Today, when we speak of a 'farm' as a piece of land where crops grow and animals graze, we are using a word whose entire etymological history is about financial contracts, not agriculture.
Modern Legacy
This history leaves visible traces in modern English. 'Pharma' and 'farmer' are frequently confused as related, but they are etymologically unrelated: 'pharmacy' comes from Greek 'phármakon' (drug, remedy), while 'farmer' comes from Latin 'firmus.' The similarity is pure coincidence. Conversely, the connection between 'farmer' and 'firm' is genuine but almost never noticed — the farmer and the law firm are etymological siblings, both named for the concept of a fixed, settled arrangement.