Similar to a third wheel, but instead of one couple and a single person, there are two couples and a single person. The name comes from the fifth wheel in a car, or the spare tire.
It was supposed to be a double date, but Tim decided to show up, thus making himself a spare tire.
the girl you are talking to for no other reason than in case all your other ho-contacts suddenly turn cold on you. the spare tire in this case would be installed and ready to satisfy your sexual desires
Greg: Man, why you talkin' to Juanita? She's fuckin nasty!
Harold: She's my spare tire, you know, just in case...
The "ring" of fat wrapping around the mid-section, caused by lack of self-respect and being abused as a child. -used to roll oneself down hills when tired.
Hey you there, I justgot a flat, can I borrow that spare tire on your waist?