When you're a morbidly obese man who likes even more morbidly obese women, but you're both too fat to copulate. So instead, you have her sit on your face. However, because she is so morbidly obese, she can't reach far enough around to wipe herself, so you end up with a vertical racing stripe of feces on your face. The dirty DeVito.
They were so fat, the best they could do is a Dirty DeVito. Afterwards he had to wash his face.