know (one's) onions(redirected from know his 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.
know your onionsBRITISH, 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'.
know your onionsbe fully knowledgeable about something. informal
Onions is perhaps short for rhyming slang onion rings , meaning ‘things’. The phrase dates from the 1920s.