1A unit of weight, used (especially formerly) in Spain, Portugal, and South America, varying locally from approx. 24 to 35 pounds (11 to 16 kg); specifically (a) a unit of weight used in Spain and some (former) Spanish colonies, equal to 25 Spanish pounds (11.5 kg); (b) a unit of weight used in Portugal and some (former) Portuguese colonies, equal to approx. 32 pounds (14.5 kg).
2A unit of liquid measure, used (especially formerly) in Spain and some (former) Spanish colonies, varying locally and according to the liquid, from approx. 2.6 to 3.6 gallons (11.8 to 16.4 litres).
Mid 16th century; earliest use found in Richard Eden (c1520–1576), translator. From Spanish arroba, †arrova from Arabic al-rubʿ from al- the + rubʿ quarter (from the same base as arbaʿa four), the unit of weight being so named because it equals one-quarter of the Spanish quintal.