"For weeks Armstrong, guided by Luis, followed his
regular route among the mountains. After he had col-
lected an arroba of the precious metal, winning a profit
of nearly $5,000, the heads of the lightened mules were
turned down-trail again. Where the head of the Guarico
River springs from a great gash in the mountain-side,
Luis halted the train."


Webster's 1913 Dictionary

Definition: \Ar*ro"ba\, n. [Sp. and Pg., from Ar. arrub, ar-rubu, a
fourth part.]
1. A Spanish weight used in Mexico and South America = 25.36
lbs. avoir.; also, an old Portuguese weight, used in
Brazil = 32.38 lbs. avoir.

2. A Spanish liquid measure for wine = 3.54 imp. gallons, and
for oil = 2.78 imp. gallons.

From Arabic rub’a 'a weight of 12.5 kilos'.