/ˈɑːrmi/
[noun] A large, organised group of soldiers trained to fight on land, or more generally, any very large group of people or things working together toward a common goal.
The word 'army' comes from a Latin root meaning 'to arm' — making every army, at its heart, simply a collection of armed people.
The story of 'army' begins in Latin, with the verb 'armare', meaning 'to arm' or 'to equip with weapons'. This verb was itself built from the Latin noun 'arma', meaning 'weapons' or 'tools of war' — a word that Roman soldiers would have heard every day of their military lives. From 'armare' came the past participle 'armata', meaning 'armed' or 'equipped', which was used as a shorthand for a force of armed people.
As Latin evolved into the family of Romance languages during the early medieval period, 'armata' began its journey westward and northward. In Old French, the word became 'armée', the feminine past participle of the verb 'armer' (itself directly descended from Latin 'armare'). The feminine form was used because the implied noun — 'force' or 'troupe' — was grammatically feminine in French. 'Armée' referred specifically to a large body of armed soldiers organised for war, and it was this French word that crossed the English Channel.
English adopted 'armée' in the late 14th century, spelling it 'army' and smoothing it into the patterns of Middle English. The earliest recorded uses in English appear around the 1380s, at a time when French remained the language of the English court, aristocracy, and military command. It was entirely natural, then, that words for organised warfare would arrive wearing French clothes. The word slotted neatly into a language already filling up with French military vocabulary — 'battle', 'captain', 'siege', and 'soldier' all made similar journeys.
Interestingly, the same Latin root 'arma' also gave English a remarkable number of other familiar words, each travelling along slightly different paths. Latin 'arma' → Old French 'arme' → English 'arm' (in the sense of a weapon). Latin 'armatura' → Old French 'armeure' → English 'armour'. Latin 'armare' → Old French 'desarmer' → English 'disarm'. And further afield, Latin 'arma' → Spanish 'armada' (a fleet of armed ships), which English borrowed directly in the 16th century when Spain's famous fleet attempted to invade England in 1588.
Over the centuries, 'army' expanded beyond its strictly military meaning. By the 17th and 18th centuries, writers were using it to describe any enormous, organised mass — an army of ants, an army of workers, an army of opinions. This figurative use reflects how deeply the image of vast, coordinated numbers had embedded itself in the word. Today, whether you are talking about a national military force or an overwhelming crowd of volunteers at a charity event, the word 'army' carries with it more than six centuries of English use and two thousand years of Latin heritage — all rooted in the simple, ancient idea of picking up a weapon.