fifth wheel

Meaning

Noun

• A type of trailer hitch, which consists of a horseshoe-shaped plate on a multidirectional pivot, with a locking pin to couple with the kingpin of a truck trailer.
• A horizontal wheel or segment of a wheel above the front axle and beneath the body of a carriage, forming an extended support to prevent it from overturning.
• Anything superfluous or unnecessary.

Origin

• From the fact that since many vehicles such as carriages, coaches, wagons, prime movers, and trucks generally have four wheels on which they move, this component which resembles a wheel is the fifth.
• Sense 3 refers to the fact that a fifth wheel is not needed for a four-wheeled vehicle to operate.