When a friend has passed out you proceed to spread his/her butt cheeks and insert the makings of a taco (tomato, lettuce, choice of meat, sour cream, etc.) Often great for retaliation.
Eating a combination of ground beef (or seasoned grilled chicken), cheese, lettuce, and tomato (with additional dressings and sauces to be used as needed) out of the crack of another's anus.
We were all out of shells last night, but had all the other ingredients ready. I didn't want to ruin Billy's second fiesta, so we just ate butt tacos out of Melinda.