know your stuff
Meaning: If you know your stuff, you're very good at what you do, and you know a lot about it.
- The technician who looks after the computers at our office really knows her stuff. She's great!
- When it comes to growing vegetables, Uncle Ken sure knows his stuff. He's been growing veggies in his back garden for forty years!
We'll soon know how good he is at fixing cars. If he knows his stuff, the car should