fifth wheel

Short definition from Wiktionary

Noun

1. 1
   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.
2. 2
   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.
3. 3
   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.
4. 4
   Anything superfluous or unnecessary.