A sex act where a man gets on his hands and knees while the girl strokes his penis from behind. At the same time, she tosses the guy's salad. During that time, the guy lets loose a load of diarrhea, creating the "mudslide."
Note: Contaminated water in Guatemala lends itself to diarrhea production.
I gave the girl a Guatemala Mudslide. It was great, but the cleanup is a bitch.