After taking a shit, you don't not manage to wipe all shit residue. Then later in the day, while doing an activity that causes you to sweat... your left over shit residue and sweat combine, leaving you with swap ass. It itches and smells terrible. Bascically it becomes itchy ass butt sweat.
While playing basketball, Richard felt an errie feeling of swap ass, his left over shit residue combined with sweat. He had to leave immeadiately to take care of his situation.