fifth wheel

基本解释

The term “fifth wheel” comes from a part that was often used in carriages.
术语“多余的人或物”（字面意思：第五轮）来自经常使用四轮马车的一部分
All the boys and girls were dancing at the party while I sat at a table alone, feeling like a fifth wheel.
晚会上，所有的男孩和女孩都在跳舞，只有我孤零零地坐在桌边，感觉自己仿佛是一个多余的人。
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By definition, a fifth wheel is a wheel or a portion of a wheel with two parts rotating on each other that sits on the front axle of a carriage and adds extra support so it doesn’t tip.
通过定义，第五个轮子，是成为一个四轮马车的前车轴上互相旋转的两个部分的一个轮子或一个轮子的一部分，而加上一个额外的辅助轮子，这样它就不会倾斜。

Ted Scott, the American Trucking Association's director of engineering, said an automatic locking device secures the pin into the fifth wheel.
NPR: Runaway Trailer Crashes Rare; NY Accident Killed 7
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.
NPR: Runaway Trailer Crashes Rare; NY Accident Killed 7
"It is possible that with an old, not properly maintained fifth wheel that the device can get loose, the sides can wear away, the hole that the pin fits into is no longer snug and rigid, " Scott said.
NPR: Runaway Trailer Crashes Rare; NY Accident Killed 7