Your loin is the part of your body just above your hip, but below your rib cage. A low-slung belt hangs more or less along your loin.
Loin often refers generally to a person's body below their ribs, though rarely by doctors (who like to get more specific). At the butcher, though, it means a particular cut of meat, usually beef or pork, from an animal's low back and side. This is the earliest meaning of the word, which comes from the Old French loigne, "hip or haunch." In the Bible, loin means "part of the body that should be covered with clothing."