The etymology of this word is unclear, and the OED (Third Edition, 2011) questions any relation with Spanish arroba (a measure of dry goods or liquids) on the basis of ‘serious phonological and semantic difficulties’.
s.
1sort of keg or cask, container:
(
1365-71
)
in uno pari de rebbours de novo fact’
i 178
(
1404
)
1 parva cista pro garcione [...] 2 par’ de rubbours
ii 397
rubbour (1365-71)