Pronunciation: (un'yun),[key] —n. 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. 6. know one's onions,Slang.to know one's subject or business thoroughly; be capable or proficient.
—adj. 1. containing or cooked with onions: onion soup. 2. of, pertaining to, or resembling an onion.