The '@' symbol, essential for email, has an origin long before the Internet. From what medieval Latin word does its curious Spanish name 'arroba' come?

The name 'at' for the @ symbol comes from the Hispanic Arabic 'ar-rub', which in turn comes from the classical Arabic 'rub', meaning 'fourth'. In the Middle Ages, it was used as a unit of weight and volume (equivalent to a quarter of a 'quintal', about 11.5 kg). Merchants wrote it as an 'a' surrounded by a curved line, which over time evolved into the current symbol. Ray Tomlinson chose it for email in 1971 because it was a rarely used character that would not cause confusion.