fifth wheel

基本解释

The term “fifth wheel” comes from a part that was often used in carriages.
術語“多餘的人或物”（字麪意思：第五輪）來自經常使用四輪馬車的一部分
All the boys and girls were dancing at the party while I sat at a table alone, feeling like a fifth wheel.
晚會上，所有的男孩和女孩都在跳舞，衹有我孤零零地坐在桌邊，感覺自己倣彿是一個多餘的人。
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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.
通過定義，第五個輪子，是成爲一個四輪馬車的前車軸上互相鏇轉的兩個部分的一個輪子或一個輪子的一部分，而加上一個額外的輔助輪子，這樣它就不會傾斜。

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