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[uhn-yuh n] /ˈʌn yən/

noun
1.
a plant, Allium cepa, of the amaryllis family, having an edible, succulent, pungent bulb.
2.
any of certain similar plants.
3.
the bulb of the onion plant.
4.
the flavor or odor of this bulb.
5.
Slang. a person:
He’s a tough onion.
adjective
6.
containing or cooked with onions:
onion soup.
7.
of, relating to, or resembling an onion.
Idioms
8.
know one’s onions, Slang. to know one’s subject or business thoroughly; be capable or proficient.
/ˈʌnjən/
noun
1.
an alliaceous plant, Allium cepa, having greenish-white flowers: cultivated for its rounded edible bulb
2.
the bulb of this plant, consisting of concentric layers of white succulent leaf bases with a pungent odour and taste
3.
any of several related plants similar to A. cepa, such as A. fistulosum (Welsh onion)
4.
(Brit, slang) know one’s onions, to be fully acquainted with a subject
adj.

1838, from onion + -y (2). Related: Onioniness.
n.

early 12c., from Anglo-French union, Old French oignon “onion” (formerly also oingnon), and directly from Latin unionem (nominative unio), colloquial rustic Roman for “a kind of onion,” also “pearl” (via notion of a string of onions), literally “one, unity;” sense connection is 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 kremuse.

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 is attested from 1952.

Onion dome attested from 1956; onion grass from 1883; onion skin as a type of paper from 1892. Onions, the surname, is attested from mid-12c. (Ennian), from Old Welsh Enniaun, ultimately from Latin Annianus, which was associated with Welsh einion “anvil.”

noun

The head (1890+)

Related Terms

know one’s onions, off one’s noodle

The Israelites in the wilderness longed for the “onions and garlick of Egypt” (Num. 11:5). This was the _betsel_ of the Hebrews, the Allium cepe of botanists, of which it is said that there are some thirty or forty species now growing in Palestine. The onion is “the ‘undivided’ leek, _unio_, _unus_, one.”

see: know one’s stuff (onions)