A fork of Rural Dictionary
Depending on the situation there's tons of ways to define what a third wheel is! A third wheel typically is person that tags along with two others, usually they are ignored or unwanted. This also can be defined as a person that serves no purpose or has no use in a situation.
She keeps tagging along on our adventures. She's just a third wheel.
We're doing all the work, why does she get to sit around and be a third wheel?