Nowadays used contemptuously or at least disrespectfully of a person or thing as useless or needless as an extra wheel for the support of a vehicle ordinarily running on four.
But in original use the term “fifth wheel” was applied to a metal wheel or circular plate (sometimes a segment of one) having an important service and never touching the ground.
It lay horizontally beneath the forward part of the body of a wagon or carriage and was attached to the upper side of the front axle, thus supporting the body while the vehicle turned a corner or the like.
