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 trailer (fifth-wheel trailer) that 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.