fifth wheel
noun
chiefly N. Amer. <主北美>
1
- an extra wheel for a four-wheeled vehicle（四轮机动车的）备用轮。
1.1
- informal a superfluous person or thing〈非正式〉多余的人（或物）, 累赘。
2
- a coupling between a vehicle used for towing and a trailer（挂车的）连接轮。
2.1
- （亦作fifth-wheel trailer）a trailer with accommodation for camping out（有生活起居设施的）野营挂车。
2.2
- historical a horizontal turntable over the front axle of a carriage as an extra support to prevent its tipping〈史〉（马车前车轴上的）转向轮。