"fifth wheel : (road transport) A type of trailer hitch, which consists of a horseshoe-shaped plate..."

“fifth wheel : (road transport) A type of trailer hitch, which consists of a horseshoe-shaped plate on a multidirectional pivot, with a locking pin to couple with the kingpin of a truck trailer.
In full, fifth-wheel trailer: a large caravan or travel trailer that is connected to a pickup truck for towing by a hitch similar to the one described in sense 1 located in the center of the truck’s bed.
(road transport, historical) A horizontal wheel or segment of a wheel above the front axle and beneath the body of a carriage, forming an extended support to prevent it from overturning.
(idiomatic, informal) Anything superfluous or unnecessary.”

- https://en.wiktionary.org/wiki/fifth_wheel#English

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