The real McCoy means the original artilce, the genuine product or the 'real thing.'
There are a number of disputed theories about its origin but the one I prefer relates to the 1920's and prohibition in America. Rather than stopping people from drinking alcohol, it resulted in organised crime, illegal clubs and some rather chemical and dangerous, homemade hooch.
However, one entrpreneur by the name of Captain McCoy had a boat and smuggled in genuine rum from Canada. This was known as 'The real McCoy.'
Yeah, the real McCoy and all that jazz!