|1.||a spare wheel for a four-wheeled vehicle|
|2.||a. the coupling table of an articulated vehicle|
|b. a steering bearing that enables the front axle of a horse-drawn vehicle to rotate relative to the body|
|3.||a superfluous or unnecessary person or thing|
A hanger-on; a person who serves no function: “The vice president felt like a fifth wheel after his exclusion from the committee.”
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."