I was watching a video on YouTube last night where they mentioned that there's a difference between the RV travel trailer type and fifth wheel. I was thinking about it today and realized that didn't make much sense.

There's no circumstance in which a fifth wheel is only going to have one more wheel (the fifth), and most of the time they have 4 more wheels on two axles, which makes it seem like they'd be 8th wheel or something.

Where does the term "fifth wheel" originate?

  • This answer clearly explains that "fifth wheel", meaning "an extra and unnecessary person or thing", goes back to the mid-1800s. This is well before the invention of the sort of semi-trailer that would employ the trailer hitch. It's reasonably likely that the trailer hitch sense derived somewhat ironically from the "unnecessary" sense (plus, of course, the physical resemblance of the hitch to a wheel). – Hot Licks Nov 29 '16 at 1:10

I think the term fifth wheel is a term for something superfluous, akin to an extra wheel on a four-wheeled carriage. The Wikipedia claim about the fifth wheel referring to the the fifth-wheel coupling on trailers seems far fetched, since it may be fifth, but it's by no means useless.

I offer as evidence a play called The Fifth Wheel, A Comedy in Three Acts, published in 1869 and owned by the De Witt Publishing House. This appears to be a comedy of manners about love and wealth, and its final lines (addressed to the audience) are as follows:

Vande. It’s a blessing you’re all so happy that you forget Joralemon and me. Poor fifth wheels! utterly useless1 — but we still form part of the family coach2.

Joral. By pairs1 both wheels and human beings move,

Mrs. B. The dual nature of our lives to prove,

Cook. We share the downhill,

Miss Sp: Uphill roads of life,

Vande. Joined by the axle Love, true man and wife!
             The Fifth Wheel takes the dust, the mire, the sun3

Brain. But for the lesson’s sake, grant it its run4!

  1. Useful wheels are paired, but the unpaired fifth wheel is "utterly useless", unlike a fifth-wheel coupling
  2. Fifth-wheel couplings are for vehicles that haul freight, not for a "family coach".
  3. Fifth-wheel couplings don't take dirt, mud, and sun; they're covered by the trailer.
  4. Fifth-wheel couplings cannot be granted a "run"; they're fixed couplings and don't turn.

Is there a reference to fifth wheel used this way before Moby Dick (1851)? Elijah, chapter 19 to the Pequot crew (referring to whether the crew had signed away their souls):

'Oh, perhaps you hav'n't got any," he said quickly. "No matter though, I know many chaps that hav'n't got any — good luck to 'em; and they are all the better off for it. A soul's a sort of a fifth wheel to a wagon."

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