In the legend of the bitter feud between the Hatfields and McCoys, a bloodline offspring of the McCoy's was the real McCoy. A child born from parents consisting of one Hatfield and one McCoy was not a real McCoy.
Therefore anything that is not genuine is not the real McCoy.
This store brand coke is not the real McCoy.
Oilers on early trains had to be oiled to stop friction on the drive wheels. Oilers always failed... except the McCoy brand and they became known as the best and 'the real McCoy'
These appear to be the real McCoy.
The genuine article.
William S. (Bill) McCoy was a bootlegger known for selling top quality liquor that was not watered down, i.e. the "real McCoy".
1. The real deal.
2. A brand name for chips in Britain/Europe.
1. "I dont think that guys a fake.. He's the real mccoy!"
2. HEY YOU! Pass me a bag of Mccoys.