know one's onions
/nəʊ wʌnz ˈʌnjənz/
idiom
Origin
English
onions — Expertise, ability, or trivial knowledge.
Overview
Meaning Now
- To be very knowledgeable or experienced in a particular field, task, or subject.
Language path
English
Earliest form
know his onions
Earliest period
19th century
Journey
The phrase is characteristically British slang. While some suggest it refers to the complex layering of an onion—implying that one knows the details of a matter 'layer by layer'—others attribute it to early 20th-century American or British entrepreneurs who used 'onion' as a playful term for something of value or a person of consequence. It shares a period of origin with similar 'know one's...' expressions like 'know one's stuff' or 'know one's oats'.
Evolution
- 1890s
to know one's onions
“To be clever or well-informed.”
Attested in slang dictionaries around the end of the Victorian era.
- 1920s
know one's onions
“To possess practical expertise.”
Became widely used in British colloquial speech.
Connections
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External references use DefinedTerm semantics.