5th wheeling

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.