fifth wheel

noun5

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. •
   (US, informal) Anything superfluous or unnecessary.

   • I felt like a fifth wheel when both of them started giggling and making out during dinner.
3. •
   (US, informal) An unwanted person accompanying two couples on a double date.
4. •
   (US) 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.

   • Fifth-wheel hitching is a simple affair compared to trailer coupling. Choose a quality hitch and don't overburden an inadequate pickup truck with a big fifth wheel and you're almost in business. — 1997
5. •
   (US, 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.

   • For greasing the axles and fifth wheel, use castor-oil. It is not necessary to put on a great deal. [...] The fifth wheel should be looked after, and not be allowed to become entirely dry. — 1875