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1.
When the fatty tissue of an individual's lower back is pressed together by the waste belt of their trousers, giving the appearance of an abnormally long anal cleft, or "Bonus Butt".

The phenomenon is frequently observed when an overweight individual leans forward while wearing a short T'shirt. Bonus Butt sightings are often reported at Ikea and other discount furniture stores, where the obese can be seen plopping and rolling around on various furniture items.
"Gross. I just got an eye full of Tammy's Bonus Butt."

"Don't bother. Nothing in there but boomachs and Bonus Butts"

"That is a tasty Bonus Butt. Yum yum yum."
by Dalton_Road_House October 09, 2012