know one's stuff(redirected from know one's onions)
know (one's) stuff
To be very knowledgeable or skilled in some area. I think Nicole is our best candidate for the job—she really knows her stuff when it comes to corporate accounting.
know one's stuff
Also, know one's onions. Be experienced or knowledgeable in one's field or in the matter at hand. For example, Patrice knows her stuff when it comes to Mexican history, or We need a handyman who knows his onions. The allusion in the variant is unclear. [First half of 1900s]