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Phrase(s): a fifth wheel
Fig. an unwelcome or extra person. • I don’t like living with my son and daughter-in-law. I feel like a fifth wheel. • Bill always begs to come on camping trips with us, but really, he’s a fifth wheel.
McGrawhill's American Idioms And Phrasal Verbs
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An extra and unnecessary person or thing, as in He was the only one without a date, so he felt like a fifth wheel. This expression, which alludes to an unneeded wheel on a four-wheel vehicle, may have originated as long ago as 1631, when Thomas Dekker wrote Match Me in London: "Thou tiest but wings to a swift gray Hounds heel, And addest to a running Chariot a fifth wheel."
American Heritage Idioms