fifth wheel

noun
1.
a horizontal ring or segment of a ring, consisting of two bands that slide on each other, placed above the front axle of a carriage and designed to support the forepart of the body while allowing it to turn freely in a horizontal plane.
2.
a similar device used as a coupling to connect a semitrailer to a tractor.
3.
a similar coupling between a heavy-duty pickup truck and a camping trailer (fifth-wheel trailer) that extends over the bed of the truck.
4.
an extra wheel for a four-wheeled vehicle.
5.
a superfluous, unneeded, or unwanted person or thing: They enjoy making any outsider feel like a fifth wheel.

Origin:
1870–75

Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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Collins
World English Dictionary
fifth wheel
 
n
1.  a spare wheel for a four-wheeled vehicle
2.  a.  the coupling table of an articulated vehicle
 b.  a steering bearing that enables the front axle of a horse-drawn vehicle to rotate relative to the body
3.  a superfluous or unnecessary person or thing

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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American Heritage
Cultural Dictionary

fifth wheel definition


A hanger-on; a person who serves no function: “The vice president felt like a fifth wheel after his exclusion from the committee.”

The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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American Heritage
Idioms & Phrases

fifth wheel

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 American Heritage® Dictionary of Idioms by Christine Ammer.
Copyright © 1997. Published by Houghton Mifflin.
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Example sentences
Towing a fifth wheel trailer with a long bed truck increases the turning
  radius, but you might.
Fifth wheel halves, not securely attached to vehicle.
The end of the kingpin is even with the top of the fifth wheel.
The slope angle of the road was measured by a sensitive electronic pendulum,
  and the distance was measured by the fifth wheel.
Synonyms
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