from Wikipedia - "The term 'fifth wheel' comes from a similar coupling used on four-wheel horse-drawn carriages
. The device allowed the front axle assembly
to pivot in the horizontal plane, to facilitate turning. Basically a wheel was placed on the rear frame section of the truck, which back then only had four wheels, this wheel that was placed on the frame was the "fifth wheel" hence the name. The trailer needed to be raised so that the trailer's pin would be able to drop into the central hole of the fifth wheel." Fast forward to todays world and the fifth wheel would be mounted in the bed of the pick-up, although it doesn't look so much like a wheel anymore.