Were did the word fifth wheel originate from?

fith wheel on a truck originated on a 4 wheel vehicle as an extra wheel

the first 5th wheel or turntable was a fixed unit and was not a quick release

this wheel had bearings between 2 plates that were bolted together,(sort of like a big metal sandwich)and then bolted to both tractor and trailer.

to separate the two, it had to be ither unbolted from the truck or trailer,but this was only done in the need of repairs or permanent replacement