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.
A sex act in which one partner inserts their hand and arm into the other partner's rectum, up through their stomach and intestine, and into their mouth through their throat, at which point they use their hand to open and close the partner's mouth much like a manequin. Usually practiced during necrophilia. Named after Danny Devito, who is of manequin size.
Last night, I dug up your grandma and gave her the dirty Devito.