“fifth wheel”

This is an expression mostly used in American English. A fifth wheel is a person or a thing that is not required.
In other words, the person is the unwanted extra. He is like the fifth wheel of a car that already has four good ones, and is therefore unnecessary
Example: “The team already has two very good off spinners. I feel like a fifth wheel.”

“It is also possible to say, `third wheel’.”
Example: “Lavanya is just a third wheel around here. Why don’t you send her home?”

Source: ‘Know Your English’ ( The Hindu) – June 11, 2007