But it's superfluous, really-which is why calling someone a "fifth wheel" is a way of calling them unnecessary, basically a tagalong.

但它是多余的，真的- - -这就是为什么将一个人称作“第五轮”是指他们无用，根本是个跟屁虫。

youdao

He was the only one without a date, so he felt like a fifth wheel.

他是唯一没被邀请的人，因此他感到自己很是多余。

youdao

The team already had two goalkeepers, so I felt like a fifth wheel.

队里已经有两个守门员了，我完全是多余。

youdao

Trailers are typically attached to tractors by a "fifth wheel, " in which a coupling pin on the trailer fits into the center of a large horseshoe-shaped piece of metal.

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