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

[TahlilGaran] American Dictionary ▲

a fifth wheel
[American]
someone who is in a situation where they are not really needed or are ignored by other people.
I don't have a role in the office any more - I feel like a fifth wheel.

[TahlilGaran] English Idioms Dictionary ▲