A person hanging out with two people (usually a couple) that are in a sence leaving him/her out. Called the third wheel because either:
-Having any odd number of wheels on an object generally messes it up.
-Third wheel on a bike makes a tricycle, something thats safe for kids be left alone with.
Well, if your bringing your girlfriend, I'll have to take my cousin with me, don't wanna be the third wheel.