Fifth wheel

《略式》無用の長物,よけいな人[物].


(eg) Jim: "I'm gonna see a movie this weekend."  
(週末は、映画観るよ)

Brother: "Really? Can I go with you?"
(ほんと?一緒に行っていい?)

Jim: "Noooooo, I'm going with Kate. You don't want to be a fifth wheel, do you?"
(ダ~メ!ケートと行くから。邪魔者になりたくなりだろ?)