Third wheel idiom
The word "third" can be subtituted by "fifth" without changing the meaning.
Used to refer to a person who is unnecessary in a group
Every time hanging out with Rachel and Ross, I feel like a third wheel.
When accidentally walking into a restaurant for couples only, I as soon realized I was a third wheel there.
I'm an introvert, so I feel like a third wheel among my classmates and struggle to find like-minded peers
Honestly speaking, nobody wants to be a fifth wheel.
This phrase may come from the idea of a two-wheeled trolley. when being added an extra wheel, that wheel seems to be spare and unnecessary.
Hide the secret instead of dealing with problems.
The city management team's attempts to sweep the scandal under the carpet were not very effective.