A person you invite on a date not because they like you but, because if you don't they'll be the Third Wheel. After this date they'll most likely never talk to you again and push you to the side.
Henry:Yeah I finally am going out on a date with Jamie.
Andy:Dude, I wouldn't be so happy the only reason she's doing this is because she doesn't want to be the Third Wheel.
Henry: So I'm just a..
Andy: Yep sorry dude your just a Spare Wheel.