A "fifth wheel" refers to a person who is unnecessary or unwanted in a social situation, often feeling out of place. The term originates from the extra wheel on a vehicle that is not needed for it to function properly, symbolizing someone who does not contribute to the dynamics of a group. In social contexts, a fifth wheel can occur in various scenarios, such as when a couple invites a single friend to join them. This friend may feel awkward or excluded, similar to how a fifth wheel on a vehicle serves no essential purpose.