Donut on a car

Whenever a group of 5 people, wherein 4 of them paired up (as two couples or two people going for sex) and the fifth person is fifth wheeling. No longer shall they be called a fifth wheel rather a donut.

Joe: Hey man did you see the group walking over there?
Bob: yeah what about them?
Joe: the dude in the back is definitely the donut on a car tonight .
Bob: lmao.