- an extra wheel for a four-wheeled vehicle.；a superfluous person or thing.
noun: fifth wheel, plural noun: fifth wheels
- 1. an extra wheel for a four-wheeled vehicle.
- ▪ informal a superfluous person or thing.
- 2. a coupling between a vehicle used for towing and a trailer.
- ▪ a trailer with accommodation for camping out.
- ▪ historical a horizontal turntable over the front axle of a carriage as an extra support to prevent it tipping.