know stuff(redirected from know onions)
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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 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.