early 12c., ungeon, oinyon, unione, "the underground bulb of the common onion plant," from Anglo-French union, Old French oignon "onion" (formerly also oingnon), and directly from Latin unionem (nominative unio), a colloquial rustic Roman word for a kind of onion, also "pearl" (via the notion of a string of onions), literally "one, unity." The sense connection is the unity of the successive layers of an onion, in contrast with garlic or cloves.
Old English had ynne (in ynne-leac), from the same Latin source, which also produced Irish inniun, Welsh wynwyn and similar words in Germanic. In Dutch, the ending in -n was mistaken for a plural inflection and new singular ui formed. The usual Indo-European name is represented by Greek kromion, Irish crem, Welsh craf, Old English hramsa, Lithuanian kermušė.
The usual Latin word was cepa, a loan from an unknown language; it is the source of Old French cive, Old English cipe, and, via Late Latin diminutive cepulla, Italian cipolla, Spanish cebolla, Polish cebula. German Zwiebel also is from this source, but altered by folk etymology in Old High German (zwibolla) from words for "two" and "ball."
Onion-ring "circular segment of an onion" (especially battered and deep-fried) is attested by 1928. Onion-dome on a church-tower, etc., is attested by 1950, so called for the resemblance of shape; onion-grass, which forms tuberous nodes in its roots (also onion-couch) is from 1823; onion-skin as a type of paper (so called for its thinness, transparency, and finish, which resemble the skin of an onion) is from 1879.
Onions, the surname, is attested from mid-12c. (Ennian), from Old Welsh Enniaun, ultimately from Latin Annianus, which was associated with Welsh einion "anvil."