fifth wheel

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

An extra and unnecessary person or thing, as in He was the only one without a date, so he felt like a fifth wheel. This expression, which alludes to an unneeded wheel on a four-wheel vehicle, may have originated as long ago as 1631, when Thomas Dekker wrote Match Me in London: "Thou tiest but wings to a swift gray Hounds heel, And addest to a running Chariot a fifth wheel."

The noun has 2 meanings:

Meaning #1: an extra car wheel and tire
  Synonym: spare

Meaning #2: a steering bearing that enables the front axle of a horse-drawn wagon to rotate