(noun) Taking a poo on one side of the toilet seat, facing the wall. On a 90 degree angle from the regular pooping position (RPP). Usually in desperation, due to skid marks or pubes on either side of the toilet seat.
A close friend who feels comfortable coming into your house via the side door, probably off the kitchen. You, too, are comfortable with this friend entering your house through the side door. A side-door friend is probably one of your best friends. Conversely, front-door friends are those acquaintances you are not all that close/comfortable with.