1.nounA 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.
2.nounA 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.
3.nounAnything superfluous or unnecessary.
"I felt like a fifth wheel when both of them started giggling and making out during dinner."