The term "fifth wheel" has multiple meanings, depending on the context in which it is used.
In a general sense, "fifth wheel" can refer to something that is unnecessary or unwanted, often used as an idiomatic expression meaning "an extra or redundant part." For example, if someone says they don't want their new car because it has a fifth wheel (meaning the spare tire), then they mean they do not want the additional spare tire.
In another context, "fifth wheel" can refer to a type of trailer hitch that is used to connect a trailer to a vehicle. This device is called a "fifth wheel" because it attaches to the vehicle at a point between the first four wheels and provides support for the weight of the trailer.
Lastly, in some social settings, particularly among RV enthusiasts or those who enjoy outdoor activities such as camping or hiking, "fifth wheel" can refer to someone who joins an existing group or couple without being part of their original party. This usage is derived from the idea that a fifth wheel (the spare tire) does not fit into the standard configuration of four wheels on a vehicle and therefore causes disruption or imbalance in the overall setup.