On any normal civilian vehicle, a fifth wheel is usually unnecessary and can even hinder the operation of the vehicle. Thus arose the metaphor of the fifth wheel, often used to describe a person who doesn't really have anything important to do; they're just taking up space without contributing in any way. Kind of like a third wheel, but not quite.

A trucker once told me that the fifth wheel is the round metal thing that joins the two parts of a semi ... you know, that disc covered with evil black grease.
Maybe he was making fun of me. Who knows, he sounded a bit weird, and we were on a bus going from Pittsburgh, PA to Columbus, Ohio.

Oh, that Greyhound speeding into the night, carrying a load of human hopes, desires, love and tired bones across the empty plains, akin to life carrying our mortal selves from nothing to nothing.
Notice that the metaphor makes both Pittsburgh and Columbus into nothing, which you may not necessarily agree with.

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