"Fifth wheel" reminded me of Robertson Davies' inventive title Fifth Business
, an amazing novel by a great Canadian writer. Davies himself explained the meaning of the title this way:
"Those roles which, being neither those of hero nor Heroine, Confidante nor Villain, but which were none the less essential to bring about the Recognition or the denouement were called the Fifth Business in drama and Opera companies organized according to the old style; the player who acted these parts was often referred to as Fifth Business.
—purportedly Tho. Overskou, Den Danske Skueplads
Davies was pressured by his publisher to provide some clear idea of what exactly "Fifth Business" was, and so Davies affixed this opening quotation, which was taken at face value. Only in 1979, when the book's Norwegian translator failed to find the citation, did Davies admit it was his invention."