a third wheel电灯泡、多余的东西。这个习语起源于17世纪的马车。四轮马车会多准备一个轮胎，以备不时之需，称为fifth wheel，当时也有两轮的轻便马车，third wheel的概念就是由此引申而来，取其“多余的”概念，来比喻“打扰人约会的电灯泡”。

【例句】
You two go ahead to the movie without me. I don't want to be a third wheel.
你们两个自己去看电影吧，我不想当电灯泡。

Larry and Jenny asked me to go out for dinner yesterday, but I said no, because I didn't want to be a third wheel.
赖瑞和詹妮昨天叫我跟他们一起出去吃饭，但是我没去，我可不想当电灯泡。