fifth wheel

From the fact that since many vehicles such as carriages, coaches, wagons, prime movers, and trucks generally have four wheels on which they move, this component which resembles a wheel is the fifth. * (anything superfluous or unnecessary): Sense 3 refers to the fact that a fifth wheel is not needed for a four-wheeled vehicle to operate.

• "I felt like a fifth wheel when both of them started giggling and making out during dinner."
• "A subjunctive mode should no more exist in the English grammar, than a fifth wheel be given to a wagon."
• "Why I'm of no more use in my own house, than a fifth wheel would be to a wagon."
• "Compressed and concentrated, confined to a single sharp pang or two, but none the less in wait for him there on the Euston platform and lifting its head as that of a snake in the garden, was the disconcerting sense that "respect," in their game, seemed somehow—he scarce knew what to call it—a fifth wheel to the coach."
• "He said his name was Sheridan, Captain Sheridan, and that he was a sort of headquarters quartermaster, to look after the staff comforts. He did not seem to have a very exalted opinion of his duties, rather regarding himself as a fifth wheel."
• "After the first minute of conversation with her I stopped feeling like a fifth wheel. We got on fine together. Of course I don't know whether I was a fifth wheel or not. There was no way I could discover just what the line-up was."
• "The most common excuse is that they [line managers] do not have enough time to train every employee and perform their other supervisory duties. In an environment such as this, a new hire often feels like a fifth wheel and begins to develop frustration and fear."
• "Hope suddenly felt like a fifth wheel. She was happy for Noah and Gillian, but seeing them together just made her feel lonelier."

• spare tool
• third wheel