fifth wheel

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.

“I felt like a fifth wheel when both of them started giggling and making out during dinner.”