know 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 when it comes to corporate accounting.See also: know, stuff

know one's stuff

 and know one's onionsto 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.See also: know, stuff