n. Person who accompanies two (or more) couples in a social situation, who does not have a partner of his or her own. Often the fifth person's attendance is encouraged by one of the well-meaning but ignorant female partners, who may feel sorry for the lonely person, but will inevitably ignore them once the lovemaking starts. It carries the connotation that the fifth person is often an unneccesary and unwelcome addition to the scene in these conditions. Derived from "the fifth wheel of a car", which needing only four wheels to run, has no need of a fifth.
"Jim and Sue, Mike and Sally invited me to come camping with them, but I refused as I would only have been the fifth wheel on my own!"