In the 1890s, a great boxer known as Kid McCoy couldn’t get the champion to fight him, and so to seem beatable, he began to throw the odd bout, and fans never knew if they’d see the “real McCoy.”
The plan worked, and he became the welterweight champion of the world.
Once, while in a bar, McCoy was challenged by a drunken patron who didn’t believe that he was the great boxer, and McCoy flattened him.
When the man came around, he declared that the man who had knocked him out was indeed the “real McCoy.”
We call the genuine article “the real McCoy”.