Origin of the Term Fifth Wheel
Wikipedia says: The term fifth wheel comes from a similar coupling used on four-wheel horse-drawn carriages and wagons. The device allowed the front axle assembly to pivot in the horizontal plane, to improve turning.
However I've also heard that the term originated with the way Glenn Curtiss designed the hitch for his fifth wheel trailer the Curtiss Aerocar Land Yacht. This origin makes more sense to me. At least as the term is used in modern day.
The "Glenn Curtiss Aero Coupler", consisted of a tire and wheel mounted horizontally to the trunk floor of a passenger coupe or to sq specially designed tow vehicle. The tire is inflated to absorb lateral, longitudinal and vertical road shocks. The trailer's goose neck kingpin slips into the wheel's bearing journal where an axle would normally fit.