know one's stuff

Once I knew stuff on my own, what was to stop me from trying to go into business for myself?

But at the same time the women knew their stuff.

Does he sound like someone who doesn't know his stuff?

This year was going to be a test of how well he knew his stuff.

I think if you know your stuff, you're going to be fine.

The guy knew his stuff, but he was too ready to show it off.

I know stuff about Sam that's going to come out.

Nothing else is known about her except that she "knows stuff".

Haven't you ever wondered how come I know so much stuff about other people?

Someone once told me that, if you knew your stuff, old walls could be read like books.