A 5th wheel on a car isn't needed,so when a person feels left out or not needed,they may refer to themselves as a 5th wheel.*+++++*

Fifth wheel originated in the days of horse drawn wagons.It was 'the 5th wheel' of the wagon.

1) "An extra and unnecessary person or thing, as in He was the only one without a date, so he felt like a fifth wheel. This expression, which alludes to an unneeded wheel on a four-wheel vehicle, may have originated as long ago as 1631, when Thomas Dekker wrote Match Me in London: "Thou tiest but wings to a swift gray Hounds heel, And addest to a running Chariot a fifth wheel."" Source and further information: http://dictionary.reference.com/browse/fifth%20wheel 2) "The term “fifth wheel” comes from a part that was often used in carriages. By definition, a fifth wheel is a wheel or a portion of a wheel with two parts rotating on each other that sits on the front axle of a carriage and adds extra support so it doesn’t tip. But it’s superfluous, really—which is why calling someone a “fifth wheel” is a way of calling them unnecessary, basically a tagalong." Source and further information: http://www.smithsonianmag.com/science-nature/A-Salute-to-the-Wheel.html?c=y&page=2 "Long ago, there was a device on cars that was a horizontal wheel attached to the front axle. This wheel wasn't really used much, except maybe as support for a sharp turn? But for the most part, it was useless. Being a fifth wheel soon came to represent anyone who was without a partner in a group that was paired-off. Just like the car had 4 wheels (2 pairs) and the 5th was just along for the ride, so-to-speak!" Source and further information: http://www.brownielocks.com/wordorigins.html "Since early sources, going back to the 17th century, testify to longer forms of fifth wheel that include the fifth wheel of a coach, carriage, waggon/wagon, etc., perhaps I could find a link between some literal fifth wheel and the figurative one--and perhaps there was no such link for third wheel. It turns out that there really was a fifth wheel, possibly of two sorts, on early horse-drawn four-wheel carriages. One was a horizontal wheel that rotated on an axle so as to help the carriage to turn--still a feature of tractor-trailer trucks today. (Since this wheel is quite useful, authors of this version of fifth wheel origins are puzzled as to why it came to be considered unnecessary.) The more interesting speculation, found in Brewer's Dictionary of Phrase and Fable, is that there was often a spare wheel, "a cumbersome object carried at the rear of the carriage, where it had to be unlashed and relashed whenever it was necessary to retrieve a piece of luggage from the storage space behind it." While that wheel wasn't superfluous in an emergency, it was certainly a nuisance. A parallel between that and a tagalong person is obvious." Source and further information: http://www.randomhouse.com/wotd/index.pperl?date=20011102

Probably from the term third wheel but just when its a double date plus 1