Per Garrison, again:

The Real McCoy - Kid McCoy was a boxer in the 1890s. He did not have a formidable appearance, and apparently was challenged as to his identity by more than one barfly. Kid McCoy's demonstrations that he was the real fighter created legends, and as a result anything which was indisputably the authentic article was called the real McCoy.

Deadlock - from wrestling, a hold which can immobilize an opponent indefinitely but is unable to force submission. Such a hold kills action, and came to apply to a stalemate of any kind.