- A type of trailer hitch, which consists of a horseshoe-shaped plate on a multi-directional pivot, with a locking pin to couple with the kingpin of a truck trailer.
- A very large truck trailer; an 18-wheeler.
- A very large trailer that is towed with a pickup truck.
- A horizontal wheel or segment above the fore axle of a carriage and beneath the body, forming an extended support to prevent careening.
- (idiomatic) Anything superfluous or unnecessary.
- As the third person along on their date, I felt like a fifth wheel.
- I felt like a fifth wheel when they started giggling and making out during dinner.
a very large truck trailer
a very large trailer
anything superfluous or unnecessary