fifthwheel

fifth wheel n.

1.

a. A wheel or portion of a wheel placed horizontally over the forward axle of a carriage to provide support and stability during turns.

b. 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.

* * *