Inside out means with the inner surface facing outward, or turned so that what's normally hidden becomes visible. When you accidentally put your shirt on inside out, the seams and tags that usually touch your skin are showing on the outside. Socks worn inside out display their fuzzy interior rather than their smooth exterior.

The phrase can also describe knowing something thoroughly and completely. When you know a subject inside out, you understand it so well that you're familiar with every detail, like knowing every level, secret passage, and hidden trick in your favorite video game. A mechanic who knows engines inside out understands not just how they look from the outside but how every internal part works together.

You might also hear someone say they've turned a room inside out while searching for something, meaning they've looked everywhere, even in unlikely spots. This connects to the same idea of revealing what's normally hidden: when you search a room thoroughly, you check inside drawers, under cushions, and behind furniture, exposing spaces usually concealed from view.