The term third wheel is a corruption of fifth wheel, which refers to someone who deters the socialization of a couple, perhaps when being invited out of pity or through a feeling of duty.
third wheel是fifth wheel（多余的人，备用胎）的误用，指的是影响一对夫妇或情侣交往的人，也就是中文里常说的电灯泡。
- -Will you go to the cinema with Jason and Sally tomorrow?
-你明天和jason和sally他们一起去电影院吗？
-No, I don’t want to be their third wheel.
-才不呢，我才不要当他们的电灯泡呢！