a fifth wheel

fifth wheel

Someone who has no real place or purpose in a situation, likened to a superfluous extra wheel on a four-wheeled vehicle. I didn't realize that the party was for couples only, so when I showed up alone, I felt like a fifth wheel.
See also: fifth, wheel
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

a fifth wheel

or

a third wheel

AMERICAN
A fifth wheel or a third wheel in a situation is someone who is not needed or wanted there. As a single person, you're somewhat of a third wheel when traveling with couples. I just wanted to feel like part of the family instead of a fifth wheel. I wanted to feel like I belonged! Note: A fifth wheel on a car or a third wheel on a bicycle would be unnecessary.
See also: fifth, wheel
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012

a fifth/third ˈwheel

(American English) an unwanted, extra or unnecessary person: No, I don’t think I’ll join you. Whenever I go out with you guys I just feel like a fifth wheel.
This refers to adding an extra unnecessary wheel to a vehicle.
See also: fifth, third, wheel
Farlex Partner Idioms Dictionary © Farlex 2017
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References in periodicals archive ?
They say help and guidance from Denbighshire County Council and securing European grant aid was crucial in setting the company on the road to success Fifth Wheel believes its Celtic Rambler - the only British-built fifth wheeler - has improved on the American classic which has a fifth wheel coupling attached to a pick-up truck David Walker, the county council's economic development officer, said: 'The fact that a company from Australia wants to import the Celtic Rambler is a major coup.