the real McCoy

Example: “Forget the imitations; this Swiss watch is the real McCoy.”

Origin: Genuinely unresolved, with four candidates and an awkward chronology. The best-known story credits the boxer Norman Selby, who fought as Kid McCoy in the 1890s and supposedly settled a skeptic's doubts about his identity with a demonstration. Others credit Elijah McCoy, the Black Canadian-American inventor whose locomotive lubricators were widely imitated, or Bill McCoy, a Prohibition-era rum-runner known for not watering his liquor. The difficulty for all three is that a Scottish form is attested earlier: G. Mackay and Company of Edinburgh advertised its whisky as the real Mackay from around 1870, and the spelling shift from Mackay to McCoy would be an ordinary piece of transatlantic drift. That does not settle it either, but it does mean the boxing story cannot be the origin, whatever it did for the phrase's popularity.