At Taco Bell, a dirty diaper consists of a sloppy, poop-like scoop of refried beans on top of a 6.5-inch flour tortilla. It is wrapped around a crunchy taco, thus completing the disgusting-looking (but surprisingly popular) item known as the "Double Decker."
"Why'd you wrap that taco? It was supposed to go in this dirty diaper!" *speaker points to a lonely bean-laden tortilla*