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 your ˈstuff
(informal) know everything that you should know about a job, a subject, etc: I was very impressed by your lawyer. He really knows his stuff.