A spotty ass also known as buttknee, or in a more descriptive way, acne on your bum that you get from sitting down. This is caused from not smoothing your bum, some are even born with it. Name is Butt Acne
An extremely obese person's knees that are exposed and create a likeness to a butt view from the layers of extra skin that dimple and come together while standing or walking or the posterior image created when an extremely obese person is standing still and wearing shorts or a skirt
Hey Man did you see the Butknees on that whalelady on the escalator as we went to baggage claim??
Yah, her knees were like a bad fat ass.......cover that up! That should be illegal.