Origins
The verb 'pay' is so thoroughly associated with money that its true origin comes as a revelation: it descends from the Latin word for peace. English borrowed the word from Old French 'paier' (modern French 'payer') around 1200 CE, and Old French had inherited it from Latin 'pācāre,' a verb meaning 'to pacify, to appease, to make peaceful.' The Latin verb was derived from 'pāx' (genitive 'pācis'), meaning 'peace, compact, agreement.' To pay, at its etymological root, was to make peace.
The semantic development from 'pacify' to 'pay money' is not as strange as it first appears. In Roman and medieval society, an unpaid debt was understood as a breach of social peace — a state of tension or conflict between debtor and creditor. To 'pacify' the creditor by settling the debt was to restore the social equilibrium that the obligation had disturbed. The Latin 'pācāre' was used in this financial sense as early as the late Roman period, and by the time Old French inherited the word, the commercial meaning was already dominant, though the older sense of 'satisfy, appease' lingered.
The Latin root 'pāx' itself has a deeper history. It derives from the PIE root *peh₂ḱ-, meaning 'to fasten, to fix,' suggesting that the original concept of peace was not the absence of conflict but the fastening of an agreement — a compact or bond between parties. This connects 'pay' to an unexpectedly large family of English words: 'peace' (via Old French 'pais' from 'pāx'), 'pact' (from Latin 'pactum,' something agreed upon), 'pacify,' 'appease' (via Old French, ultimately from 'pāx'), and even 'fang' and 'fence' through the Germanic reflex of *peh₂ḱ- meaning 'to seize, to catch.'
Proto-Indo-European Roots
The Romance cognates of 'pay' are immediately recognizable: French 'payer,' Italian 'pagare,' Spanish 'pagar,' Portuguese 'pagar,' Romanian 'a plăti' (which replaced the Latin-derived form). All descend from Vulgar Latin *pācāre with the same shift from pacification to payment. The widespread adoption of this metaphor across all Romance languages suggests that the semantic change occurred in late Vulgar Latin, before the breakup of the Roman Empire.
English 'pay' displaced no native Germanic synonym because Old English had no single dominant verb for the concept. The Anglo-Saxons used various expressions: 'gieldan' (to pay, yield — surviving in modern 'yield'), 'forgieldan' (to repay), and 'lēanian' (to reward, recompense). The Norman French 'paier' was adopted because it offered a neat, general term for a concept that Old English expressed through multiple, more specific words.
The past tense 'paid' is regular, but the alternative form 'payed' exists in the nautical sense ('to pay out a rope,' meaning to let it run out), which derives from a completely different source — Old French 'peier' from Latin 'picāre' (to coat with pitch), since ropes were waterproofed by coating them with tar. The homophony of 'pay' (give money) and 'pay' (coat with pitch) in English is pure coincidence, the collision of two unrelated Latin words.
Modern Usage
The modern English idiom is rich with 'pay' compounds and expressions. 'Pay attention' (attested from the seventeenth century) treats attention as a debt owed. 'Pay one's respects' treats courtesy as an obligation. 'Pay the price' and 'pay dearly' preserve the ancient notion that payment is a form of suffering or sacrifice. 'The devil to pay' comes from nautical usage (paying the 'devil' seam on a ship's hull with pitch), not from any Faustian bargain, though folk etymology has thoroughly fused the two images.
The word's financial derivatives have multiplied enormously in modern English: 'payroll' (seventeenth century), 'payday,' 'paycheck,' 'payoff,' 'payout,' 'payable,' 'pay grade,' 'pay dirt' (from Gold Rush mining, meaning soil rich enough in gold to be worth processing), and 'payload' (originally the revenue-producing cargo of a vehicle). Each of these compounds has moved further from the Latin root's meaning of peace-making, yet the fundamental human truth remains: we pay to restore balance, to settle accounts, to put things right between ourselves and others.