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!"
A type of RV, like a pull trailer except the hitch or pin box is over the hook up that is in the bed of a pick up truck. Like a Silverado 2500 or 3500 HD or a Superduty. A fifth wheel evenly distributes its weight over the trucks rear wheels, making the ride smoother. There are many manufactures that offer fifth wheels with different floorplans and can vary greatly in prices and lengths. You need to buy the hitch adapter for hook up after you buy the truck. It can be bolted on or welded on. Unlike pull trailers, fifth wheels will not sway.
I bought a new truck and fifth wheel to go on vacation with.