Dictionary

fifth wheel

noun

: an extra person who is not needed in a group

Full Definition of FIFTH WHEEL

1
a :  a horizontal wheel or segment of a wheel that consists of two parts rotating on each other above the fore axle of a carriage and that forms support to prevent tipping
b :  a similar coupling between tractor and trailer of a semitrailer
2
:  one that is superfluous, unnecessary, or burdensome
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First Known Use of FIFTH WHEEL

circa 1874

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