Pillow Taint occurs when a morbidly obese person sits down and the entire crotch of their pants is stretched to nearly bursting with flab.
Usually, the volume of flesh trapped in the crotch area is so great that the obese person is unable to cross (or sometimes even close) their legs.
The situation resembles a normal person with an overstuffed pillow packed down the crotch of their pants. Thus the name: Pillow Taint.
"I know... I tried to check the crotch bulge, but all I saw was a textbook case of pillow taint."