a 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.
a fifth wheelor
a third wheelAMERICAN
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.
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.