The "third wheel" refers to the person who is the outsider when there is a group of three. Example: "You two go on ahead without me. I don't want to be the third wheel." An analogy can be made to a two-wheeled bicycle or cart; adding a third wheel would not be helpful and would limit mobility. To be the "third wheel" is to be the extra, unnecessary person in a group of three people. Example: "Missy and I were going to go to the dance with another couple, but when I got sick she decided to stay home also. She didn't want to be the third wheel."