know (one's) onions

know (one's) onions

To be very knowledgeable or skilled in some area. Primarily heard in UK, Australia. I think Nicole is our best candidate for the job—when it comes to corporate accounting, she really knows her onions.
See also: know, onion
Farlex Dictionary of Idioms.

know your onions

BRITISH, OLD-FASHIONED
If you know your onions, you know a lot about a particular subject. She really knows her onions in the historical field. Note: This may derive from the rhyming slang `onion rings', meaning `things'.
See also: know, onion
Collins COBUILD Idioms Dictionary, 3rd ed.

know your onions

be fully knowledgeable about something. informal
Onions is perhaps short for rhyming slang onion rings , meaning ‘things’. The phrase dates from the 1920s.
See also: know, onion
Farlex Partner Idioms Dictionary
See also:
  • know your onions
  • know your stuff
  • know one's stuff
  • know stuff
  • know (one's) stuff
  • have a good command of
  • have a good command of (something)
  • know about
  • know about (someone or something)
  • triple threat