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fifth wheel (plural fifth wheels)

  1. A type of trailer hitch, which consists of a horseshoe-shaped plate on a multi-directional pivot, with a locking pin to couple with the kingpin of a truck trailer.
  2. A very large truck trailer; an 18-wheeler.
  3. A very large trailer that is towed with a pickup truck.
  4. A horizontal wheel or segment above the fore axle of a carriage and beneath the body, forming an extended support to prevent careening.
  5. (idiomatic) Anything superfluous or unnecessary.
    As the third person along on their date, I felt like a fifth wheel.
    I felt like a fifth wheel when they started giggling and making out during dinner.