Mackay Family Genealogy Forum
The original saying is The Real Mackay." It is Scots and refers to the various legal and illegal whisky distilleries which abounded in the Highlands of Scotland. The Mackay brothers had a legal one, which obviously made the best Scotch whisky of the time, so when drinks were being had, the taste of the good one was termed "The Real Mackay."
Americans, particularly in parts of the southwest, were not aware of the real name, and it got changed to "The Real McCoy," he apparently being a well known bull shipper in the Texas cattle country.
There is still a Mackay distillery in Scotland.