fifth wheel
Pronunciation:
/-ˈwiːl//-ˈwil/noun
Definitions
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.
Example: I felt like a fifth wheel when both of them started giggling and making out during dinner.
Synonyms:spare toolthird wheel