know your 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 about corporate accounting.
See also: know, stuff

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.
See also: know, stuff