fifth wheel

基本解釋

The term “fifth wheel” comes from a part that was often used in carriages.
術(shù)語“多余的人或物”（字面意思：第五輪）來自經(jīng)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(huì)上，所有的男孩和女孩都在跳舞，只有我孤零零地坐在桌邊，感覺自己仿佛是一個(gè)多余的人。
cid-6f31d52fcf99fd70.spaces.live.com
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(gè)輪子，是成為一個(gè)四輪馬車的前車軸上互相旋轉(zhuǎn)的兩個(gè)部分的一個(gè)輪子或一個(gè)輪子的一部分，而加上一個(gè)額外的輔助輪子，這樣它就不會(huì)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