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R76d1 TIL that "assload" and "buttload" have different origins. "Assload" originally referred to the load a donkey could carry, while "buttload" referred to the volume a "butt", a type of large cask, could hold.
YpOLA TIL 'Assload' and 'Buttload' are actual defined units of measure--the former being the maximum load of a donkey (about 8 bushels), the latter a large cask with a volume of 491 liters/108 Imperial gallons.