The backside of any person, individual. The butt cheecks, the rear end, the caboose, the ass.
Person One: Look at the large toilet sitter on that bitch.
If the 'toilet sitter' of the person is some hot babe, then, "Awww, yeah, she could sit on me any day of the week."
If the 'toilet sitter' is from a larger heavyset woman, then, "(SPIT TAKE) HOLY SHIT, WHAT? (VOMIT.)
(Not just for females, but males as well.)