Definition of "fifth wheel" []

• A wheel or portion of a wheel placed horizontally over the forward axle of a carriage to provide support and stability during turns. (noun)
• A similar device over the rear axle or axles of a tractor or pickup truck, serving as a coupling for a semitrailer. (noun)
• An additional wheel carried on a four-wheeled vehicle as a spare. (noun)
• An extra and unnecessary person or thing. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "Not being a fifth wheel for once would be a miniblessing."