A diaper is what babies wear before they’re potty trained. It’s a white cloth or other disposable fabric that’s folded up between the baby’s legs to catch the poop and whatnot. Awwww, isn’t that cute?
In many countries, a diaper is also called a "nappy.” A baby can wear a diaper, but you can also diaper a baby, which means to put a diaper on. The word diaper comes from an Old French root, diaspre, "ornamental cloth" and it used to refer to the act of putting a small pattern on a cloth that was mostly white, which is basically what babies do today.