A kitchen word with a Latin spine
An onion is the rounded, layered bulb of Allium cepa, grown for eating raw, cooked, or pickled, and also the plant that produces it. English speakers extend the same name to several related alliums, from spring onions to wild kinds. The word looks ordinary on a shopping list. Its history is not.
The origin of the word onion is a French borrowing that sits on a Latin nickname for a single bulb.
From Anglo-French into Middle English
Modern onion did not grow out of an inherited Germanic plant-name. It was taken from French. Middle English spellings scatter across the page: onyoun, oynoun, onyon, unyon, oynons. They reflect Anglo-French union and Old French oignon, itself written oingnon, ognon, and many other ways in medieval records.
Historical dictionaries do not all stamp the same first date on the English noun. The most cautious accounts treat it as a Middle English borrowing and place the earliest clear English evidence around 1356–57, in a Durham abbey account that lists onions among kitchen stores. A generation later, Chaucer’s Summoner is said to have loved garlic, onions, and leeks; the Wycliffite Bible, rendering Numbers, already has the Israelites remembering the onions of Egypt. Other summaries push the word back to the early twelfth century. Those early forms, such as ungeon, belong first to Anglo-Norman documents, one of them dated 1130. The Normans had the vegetable-name; English writers settle the later French shape in the fourteenth century. The disagreement is about which stage counts as English, not about the route.
Older English already knew a cousin of the same Latin source. Old English ynne, especially in the compound ynne-leac, is an earlier borrowing of Latin unio, built like other leek-compounds in the language. That form did not become the everyday modern word. The later French shape did.
One bulb, not a clutch of cloves
The French word comes from Latin unionem, the accusative of unio. In classical Latin unio could mean a large single pearl. In rustic speech it was also a name for a kind of onion. Both senses rest on unus, one, with the noun-forming ending that English later knows as -ion.
Why one? Sources do not tell exactly the same story, and the difference is worth keeping. A common modern explanation points to the onion’s construction: successive coats that make one bulb, unlike garlic, which splits into separate cloves. The Roman agronomist Columella, writing for farmers, says country people used unio for a variety that put forth no side shoots, so the plant stood as a single body. The pearl sense may be a second coinage from unus, because a fine pearl was worn alone, or it may be a transfer from the onion, based on shape. Some accounts even link the pearl to a string of onions. Etymologists mark that fork as unresolved. What is not in doubt is the root: the same unus that gives English one, union, unique, and unity.
The deeper ancestry is the Indo-European root reconstructed as oi-no-, one, unique. Cognates of that numeral, not of the vegetable, run through Greek, Latin, Germanic, Celtic, Baltic, and Slavic. Onion is a plant-name built on a number-word.
Two Latin onions, and a crowd of neighbors
Latin already had a more ordinary word for the vegetable: cepa (also caepa), a loan from an unknown language. That is the source of Italian cipolla, Spanish cebolla, Polish cebula, and, after a folk-etymological twist in Old High German that made the word look like two plus ball, German Zwiebel. French kept a descendant of cepa for a milder relative, ciboule, which English borrowed as chive. Old English had cipe from the same Latin stock. English onion, then, is the exception in its own family: the language took the rustic unio line through French, not the cepa line that dominates much of Europe.
The unio line did travel. Irish inniún (later oinniún) and Welsh wynwyn come from the same Latin word. Dutch ui is a back-formation: speakers treated a form ending in -n as a plural and carved out a new singular. None of these are inherited Indo-European onion words. The older European plant-name for related alliums shows up in Greek kromion, Old English hramsa (wild garlic, later called ramsons), Irish crem, Welsh craf, and Lithuanian kermušė. It is sometimes said that onion simply replaced ramsons as the general term. That is slightly too neat. Ramsons named a wild woodland allium. For the cultivated bulb, English already had ynne-leac and cipe before the French form won.
The meaning of onion in English has stayed close to the kitchen. Later compounds borrow the bulb’s look rather than its taste: onion-skin for thin translucent paper, recorded from 1879; onion-ring for a fried circular slice, from 1904; onion-dome for a church cupola, from 1950. Slang has used onion for an ounce, a baseball, a watch-seal, and, in a separate tradition, a Bermudian. Those are side paths. The core sense never left the bulb.
The surname Onions is a false friend. It is attested from the mid-twelfth century and comes from Old Welsh Enniaun, ultimately from Latin Annianus, later associated with Welsh einion, anvil. It is not a family of vegetable-growers named from the English noun.
A takeaway from the layers
If you keep one fact, keep this: English onion is a French word for a Latin one. The usual Latin vegetable-name was cepa, which still feeds Italian, Spanish, and German. French, and then English, preferred the farmers’ unio, the single bulb that does not split. Dutch speakers later misheard the ending and made ui. German speakers folk-etymologized the other Latin word into Zwiebel. The English form that stuck is the one that still sits, quietly, next to union.