From the net:
In the days of horse-drawn wagons, the fifth wheel was a wheel placed horizontally over the front axle of a large wagon to facilitate turning. Basically, it allowed the front axle to pivot.
Today, the coupler of a tractor-trailer truck is called the fifth wheel, mostly because it's round (somewhat) and is vaguely reminiscent of the wagon's fifth wheel (which much more closely resembled the wagon's other four wheels).
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