fifth wheel

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A wheel or portion of a wheel placed horizontally over the forward axle of a carriage to provide support and stability during turns.
2. n. A similar device over the rear axle or axles of a tractor or pickup truck, serving as a coupling for a semitrailer.
3. n. An additional wheel carried on a four-wheeled vehicle as a spare.
4. n. An extra and unnecessary person or thing.

Wiktionary

1. n. 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. n. A very large truck trailer; an 18-wheeler.
3. n. A very large trailer that is towed with a pickup truck.
4. n. idiomatic Anything superfluous or unnecessary.

GNU Webster's 1913

1. adj. a horizontal wheel or segment above the fore axle of a carriage and beneath the body, forming an extended support to prevent careening.

WordNet 3.0

1. n. a steering bearing that enables the front axle of a horse-drawn wagon to rotate
2. n. someone or something that is unwanted and unneeded
3. n. an extra car wheel and tire for a four-wheel vehicle