'arroba' originates from Arabic, specifically the word 'ar-rubʿ' (rubaʿ / rubaʿa), where 'rubaʿ' meant 'a quarter.'

'arroba' entered Spanish and Portuguese from Arabic during the medieval period as a term for a quarter portion (of a quintal) and became the name of a regional weight measure; later the written abbreviation/symbol associated with the measure led to use of the same word for the '@' symbol in Iberian languages and then as a loanword in other languages.

Initially it meant 'a quarter' (a fraction used in measuring weight); over time it came to denote a specific regional unit of weight (≈11.5 kg/25 lb) and later was extended to refer to the '@' symbol used in commerce and addresses.