What does it mean to say “The Real McCoy”?

What does it mean to say “The Real McCoy”?

May 09

Obviously this is an expression that means the real thing, not a substitute.  But why does one call the real thing “The Real McCoy” There is not a clear single source for this expression.  While there are examples that date back to the early and mid 19th Century, I believe that its use in modern vernacular really doesn’t date back that far. The expression, “The real MacKay”, is a...