informal to know a lot about a subject, or to be very good at doing something.
When it comes to restoring grand pianos, Mr Morley really knows his 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.
know one's stuff
and 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.