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.
To reduce friction, grease is applied to the surface of the fifth wheel.
It carries the connotation that the fifth person is often an unneccesary and unwelcome addition to the scene in these conditions.
Derived from "the fifth wheel of a car", which needing only four wheels to run, has no need of a fifth.
It was submitted for patent in 1915, and finalized in 1916.
Herman Farr invented it, and Martin hooked up with him and became the assignee. "It's a fair question whether you really can consider the fifth wheel as a milestone separate from the semi-trailer.