[ uhn-yuhn ]

  1. a plant, Allium cepa, of the amaryllis family, having an edible, succulent, pungent bulb.

  2. any of certain similar plants.

  1. the bulb of the onion plant.

  2. the flavor or odor of this bulb.

  3. Slang. a person: He's a tough onion.

  1. containing or cooked with onions: onion soup.

  2. of, relating to, or resembling an onion.

Idioms about onion

  1. know one's onions, Slang. to know one's subject or business thoroughly; be capable or proficient.

Origin of onion

1325–75; Middle English onyon<Old French oignon<Latin ūniōn- (stem of ūniō) a unity, large pearl, onion; see union

Other words from onion

  • on·ion·like, adjective
  • on·ion·y, adjective

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

How to use onion in a sentence

  • There was a certain oniony soup he had tasted there years ago whose flavor had not yet left his memory.

    A Day's Ride | Charles James Lever
  • The flower has a faint oniony smell, but fresh like box hedge.

    The Spirit of Rome | Vernon Lee
  • Everything was oniony; they finished their repast with a sweet onion-tart!

    Freaks on the Fells | R.M. Ballantyne

British Dictionary definitions for onion


/ (ˈʌnjən) /

  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

  1. any of several related plants similar to A. cepa, such as A. fistulosum (Welsh onion)

  2. know one's onions British slang to be fully acquainted with a subject

Origin of onion

C14: via Anglo-Norman from Old French oignon, from Latin unio onion, related to union

Derived forms of onion

  • oniony, adjective

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Other Idioms and Phrases with onion


see know one's stuff (onions).

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.