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The term “fifth wheel” is one of many idiomatic expressions used in everyday English, and refers to something that is not essential, or is unnecessary and useless. Paradoxically, the term supposedly originates in a useful invention in 1911 by US inventor Charles Martin of the Martin Rocking Fifth Wheel, which was a small horizontal wheel installed on trucks with attached trailers that was the primary mechanism for attaching the truck to the trailer. Early gasoline-powered trucks were actually farming tractors attached to previously horse-drawn wagons, and the fifth wheel coupler was a round plate with a central hole that Martin invented to pin the units together.
The same basic Martin fifth wheel design is still used today on modern semi-tractor trailers widely employed in over-the-road shipping. The Martin Rocking Fifth Wheel was revolutionary at the time, as it allowed trucks the ability to bend while turning corners and handle uneven surfaces without the trailer or truck wheels losing contact with the ground. It was so useful that all major truck manufacturers adopted the design.
English sayings often have difficult-to-trace origins, and its not entirely clear whether Martin's invention was the original source for the term "fifth wheel" or just the method that made the idiom a common saying in everyday language. The meaning that has been conveyed over time suggests that it may have more distant origins in the invention of the stagecoach in the western US, or even further back in time. A horse-drawn stagecoach, like a modern automobile, often carried a spare, fifth wheel along on long journeys, which served little useful purpose unless an accident involving damage to one of the primary wheels of the vehicle took place.
One type of stagecoach used for touring carried up to 18 passengers in the United States during the mid-1800s, and was actually called the 5th-Wheel Touring Wagon. Stagecoaches themselves, however, originated in the invention of the two-wheeled chariot, which can be traced back to 3000 BC in the Mesopotamia region of modern-day Iraq. Horse-drawn carriages began to proliferate in design and versatility in the 16th century in Europe, when long distance closed carriage travel became popular. Suspensions were added to make the ride more comfortable, as well as seat cushions. It may be at this time period that these vehicles began carrying spare wheels, and the term "fifth wheel" originated.
The meaning of idioms evolves over time, but the term "fifth wheel" is unique in that it appears to have retained its original context. Widespread use of Martin's invention in highway trucking has apparently had little effect on the term still retaining the connotation of something of only peripheral importance. The meaning has, in fact, come to mean more than just technology or spare parts with little day-to-day value, but also to involve the roles that people play. Someone who is said to be a fifth wheel in a company or other organization is someone who is considered unimportant, or whose duties have no great impact on current affairs and who could be easily done without.