McCoy

an authentic thing or person. Of course it's authentic. It's the real McCoy.

the real thing and not a copy or something similar Cheap sparkling wines cannot be labelled 'champagne'. It has to be the real McCoy.

Also, the McCoy. The genuine thing, as in That painting's not a reproduction-it's the real McCoy. This idiom has a disputed origin, but the most likely source is its use to distinguish welterweight champion "Kid McCoy," the name used by Norman Selby (1873-1940), from other boxers using his name to capitalize on his popularity. [c. 1900]

A long-lasting and bloody feud. The Hatfields and the McCoys were two warring families who lived along the West Virginia-Kentucky border. The 1865 murder of a McCoy, a returning Union soldier, allegedly by a band of Confederate sympathizers was attributed to a member of the Hatfield family. The death sparked some thirty years of hatred and much bloodshed between the two clans, a situation that was hardly improved when a McCoy woman ran off to live with a Hatfield who ultimately abandoned her. As word of the lengthy feud spread across the country and for years after it was settled, the two sides became a metaphor for neighborly bad blood. When, for example, two families stopped talking when one chopped down a tree on the property line between them, others in the neighborhood were likely to refer to the situation as “the Hatfields and the McCoys going at it."

The genuine article. No one is certain how “McCoy” came to stand for authenticity. It may refer to a Scottish clan leader named McKay; a prizefighter named Kid McCoy, who had a rival with the same name; or a bootlegger whose wares were what he claimed they were.