knows one's onions

to be thoroughly knowledgeable or competent in a subject area
Look at her working that poll, she really knows one's onions.
by Drewster888 October 20, 2008
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know one's onions

to be competent and authoritative in one's work
He really knows his onions when it comes distilling alcohol.(example of know one's onions)
by The Return of Light Joker October 15, 2010
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