know your 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
References in periodicals archive ?
O be flexible enough to fit into an oversized pickle jar, you've got to know your onions.
Know your onions Plant out shallots and onion sets into the garden.
And you've really got to know your onions to be successful betting on it.
EXPERTS are warning that you need to know your onions before foraging for mushrooms.
This is the perfect time of the year to get to know your onions - or more exactly their flowers.
So if you know your onions you'll pop along to your local Focus DIY store right now.
IF you're going on TV's Who Wants To Be A Millionaire, you have really got to know your onions.
GARSTON-based Can Cook wants to get Liverpool cooking, so if you don't know your onions and fancy learning how to cook in your spare time, why not join their latest campaign?
2 Know your onions - tradition tells us that Boxing Day is the best time to sow your onions.
IF you know your onions, then you will be well aware that the sparkling flower heads of alliums are part of the same humble family.