Basically is a mass unit (in Brazil and Portugal), originated at the imperial system (with an arabic name ar-arub meaning "the fourth part") of 25 pounds. The times changed, but it is still used in our metricized society for livestock weight. But adapted to a rounded number in kilos, nowadays 1 Arroba = 1 kilogram.
Well, there were other arbitrary changes along the way. I think the wikipedia article deals with it. By the time someone changed it to kg, wasn't even 25lbs anymore. Who knows who decided it would be 15kg - but that is kind the point of the oddities when change measurement systems.
This is wikipedia in portuguese for arroba through Google Translator (good enough to understand, just consider "arrogance" = Arroba): https://translate.google.com.br/translate?sl=pt&tl=en&js=y&p...
Basically is a mass unit (in Brazil and Portugal), originated at the imperial system (with an arabic name ar-arub meaning "the fourth part") of 25 pounds. The times changed, but it is still used in our metricized society for livestock weight. But adapted to a rounded number in kilos, nowadays 1 Arroba = 1 kilogram.