Know your onions

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Meaning

Be thoroughly knowledgeable or competent in a subject. The possessive normally changes with the subject: know my, your, his, her or their onions. Regional use: Now chiefly old-fashioned British English; originated in American English.

Origin

Despite its present British flavour, the expression is first recorded in the United States in 1922. It belonged to a 1920s family of playful formulas such as 'know your oats' and 'know your apples'. The theory linking it to lexicographer C. T. Onions is a later folk etymology.

Variants

  • Know one's onions
  • Know your onion

Usage Examples

  • Ask Priya about the old heating system; she really knows her onions.
  • The conservator knew his onions and identified the repair before opening the case.

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