Fifth Wheel

A standard horse-drawn wagon or carriage needed exactly four wheels to function; a fifth wheel carried along for no clear reason was simply dead weight, awkward and pointless under normal circumstances. One of the earliest documented uses of the image comes from Thomas Dekker's 1631 play Match Me in London, comparing an unnecessary addition to "a fifth wheel" on a chariot. Brewer's Dictionary of Phrase and Fable, first published in 1894, notes that while a spare wheel was cumbersome most of the time, it could genuinely save the day in an emergency, adding a little nuance to an otherwise purely negative image.