Newell's Old Boys and Rosario Central have one of the most fierce rivalries on the planet. Aside from the heated derbies on and off the pitch, this Argentinian rivalry has also provided the two clubs with curious nicknames. Newell's Old Boys are known as the "Leprosos" (Lepers, meaning someone with Leprosy) and Rosario Central are known as the "Canallas" (Bastards).
The origin of these nickname doesn't actually have an official story, but there is one that is widely accepted and repeated. Legend has it that in the 1920's a local hospital who specifically took care of people suffering from leprosy wanted to organize a match between the two clubs to raise money for their cause.
Newell's agreed and showed up for the match, while their rivals Rosario never appeared. NOB fans started chanting "canallas, canallas, canallas", to which the Rosario fans responded "leprosos, leprosos, leprosos". As the story is told, from that day on both clubs adopted the insults and proudly made them their own.
You'll find other alternative stories to the origins of these nicknames but sometimes a story is too good to not believe in it.