Before entering a vehicle, riders tend to yell out what seat they want. "Shotgun" is the front passenger.
In the back, when there are five riders, the three seats that can be called are "left nut" (left rear), "right nut" (right rear), and "penis" (middle).
However, when there are only three riders, there are no "nuts" to be called. With no "nuts" there can be no "penis" and therefore, whoever sits in the back will call fleshy patch
, or the area where the nuts and penis should of been.