know your stuff
This page is about the idiom know your stuff
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!
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Contributor: Matt Errey