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Meaning of fifth wheel

fifth' wheel'

Pronunciation: [key]
  1. a horizontal ring or segment of a ring, consisting of two bands that slide on each other, placed above the front axle of a carriage and designed to support the forepart of the body while allowing it to turn freely in a horizontal plane.
  2. a similar device used as a coupling to connect a semitrailer to a tractor.
  3. a similar coupling between a heavy-duty pickup truck and a camping trailerthat extends over the bed of the truck.
  4. an extra wheel for a four-wheeled vehicle.
  5. a superfluous, unneeded, or unwanted person or thing: They enjoy making any outsider feel like a fifth wheel.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.