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 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.