How does the old man get his nickname?

In his youth, the old man was a champion arm wrestler, and thus, was given the nickname El Campeon.

Then the negro, after the rum, would try for a tremendous effort and once he had the old man, who was not an old man then but was Santiago El Campeon, nearly three inches off balance. But the old man had raised his hand up to dead even again. He was sure then that he had the negro, who was a fine man and a great athlete, beaten. And at daylight when the bettors were asking that it be called a draw and the referee was shaking his head, he had unleashed his effort and forced the hand of the negro down and down until it rested on the wood. The match had started on a Sunday morning and ended on a Monday morning.