know (one's) stuff(redirected from know their stuff)
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 about corporate accounting.
know one's stuffand know one's onions
to know what one is expected to know. I know my stuff. I can do my job. She can't handle the assignment. She doesn't know her onions.
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]