5th wheel

Noun

fifth wheel (plural fifth wheels)

1. (US) 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 most forms of semi-trailers a great deal of time is wasted in connecting the trailer to the tractor or pulling vehicle. Most of the connecting gears are some form of fifth wheel, consisting of two horizontal circular bands of iron, one fitting on top of the other, with grease between and with a...1919 August, “Connecting the Trailer in Record Time”, in Waldemar Kaempffert, editor, The Popular Science Monthly, volume 95, number 2, New York, N.Y.: Modern Publishing Company, […], →OCLC, page 28...
   1. (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, Kim Baker, Sunny Baker, “Towing, Tow Vehicles, and Towed Vehicles”, in The RVer’s Bible: Everything You Need to Know about Choosing, Using, and Enjoying Your RV (A Fireside Book), New York...
2. (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.
   • Full fifth-wheels have become quite frequent of late, and it may not be out of place to make a few remarks on their merits as compared with half fifth-wheels. With the exception of heavy coaches and rockaways, nearly all vehicles were until lately made with half fifth-wheels; but at present we find...1873 April 1, H. F. Porter, “The Relative Merits of Full and Half Fifth-wheels”, in The Hub, volume XV, number 1, New York, N.Y.: Hub Publishing, →OCLC, page 10, column 1
3. (US, idiomatic, informal) Anything superfluous or unnecessary.
   • I felt like a fifth wheel when both of them started giggling and making out during dinner.

   Synonyms:

   spare tool, third wheel

   1. (US, idiomatic, informal) An unwanted person accompanying two couples on a double date.