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 February 23, 2009

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 18, 2010