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

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

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