Similar to the Cleveland Steamer. A guy squats down over a girls chest and defecates a huge steaming turd between her breasts. The man then "motorboats" the woman's breasts by moving his face back and forth between them and making a motor-like noise with his mouth, thus creating the Cleveland Steamboat.
"I was wondering why Mike's breath smelled shitty when he came home this morning..."
"Oh you didn't hear? He gave Caroline a Cleveland Steamboat and still had some shit on his chin!"