know what you're doing/about
know what (one) is about
To be adequately prepared or knowledgeable enough to do something; to be confident in one's abilities. You couldn't find a better leader: he knows what he's about, he stands up for his values, and he isn't afraid to put the wellbeing of others ahead of his reputation. I know her strategy may seem unorthodox, but trust me—she knows what she's about.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
know what you’re ˈdoing/aˈbout(informal) have experience of doing something and therefore understand it fully: ‘I’m worried about David using that machine.’ ‘Don’t worry. He knows what he’s doing.’
Farlex Partner Idioms Dictionary © Farlex 2017