It is the paste that is formed in the rolling folds of fat. Found mostly on the Obese. It is made up of sweat, dirt , oils, and fats. With some powder mixed in to give it a paste consistency.
I saw that enormous an woman sitting in the ER waiting room. Eating Popeyes and complaining about chest pains. It was a hot night and you could see the Duck Butter oozing out of her folds of fat that look like ribs. And she has a ring of Duck Butter under her mammaries.