fifth wheel

from The American Heritage® Dictionary of the English Language, 5th Edition.

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

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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.
• noun A very large truck trailer; an 18-wheeler.
• noun A very large trailer that is towed with a pickup truck.
• noun idiomatic Anything superfluous or unnecessary.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

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