Traditionally the dirty duty was done up forward, not back aft, hanging over the chains, at least up to the point where inside heads were invented.

The early two-castle design of merchant vessels had an outline something like a barge with a house on either end, called the forward castle and the stern castle. The poop deck's name comes from the Latin puppis, meaning stern, as Deda points out. P'raps it was the castella puppa at one point?

A