Know something inside out

Meaning

To know something inside out is to know it completely, every detail, the parts other people never see. The picture is a jacket turned inside out: you have seen the outside and the inside, the seams and the pockets. You can know a city, a job, a piece of software or a subject inside out.

It is neutral and positive, good in conversation and in job interviews. "She knows the system inside out" is a strong compliment for a coworker. Saying it about yourself sounds confident; it works best with a small proof, like how long you've done it. American speakers also say "know something inside and out" with the same meaning. A near cousin, "know something like the back of your hand", is often used for places.