Similar to the Third Wheel, the spare tire is that fifth person you bring on a double date who has to sit by themselves.
This can be especially embarrasing if the spare tire has to pull up a chair to a booth for four.
Spare tires are usually your socially awkward homies or people you feel bad for.
I'm never hanging out with Billy again. He said we were just going to the movies with 'some friends' but I actually just got played into being the spare tire!
What you say when you don't want someone around. Namely the Cedric Diggory(ies, s?) of the world. Found in Harry Potter and the goblet of fire.
and more importantly, in the internetcult favorite musical, A Very Potter musical.
Harry/Darren: 'Cedric, seriosuly, screw off. You're just this spare guy who noone wants around. You're such a spare! "
Quirrel/Voldemort/Brian/Joe : KILL THE SPARE! AVADA KEDAVERA!