In a group of friends, the one friend that is kept there as a
backup, they do just enough to keep him/her around. Only to to stuff with when the
usual crowd of friends can't. Like
the spare tire on the back of your car.
Scott: Crap I've got 3 tickets and only Jake and Connor can go,
Tanner said he couldn't go. I guess
I'll have to see if Luke can go instead since we've got no one else.
Scott: Hey Luke want to go with us?
Luke:
I guess so. Looks like I'm 5th wheeling as usual.