> 750ml is roughly 1/5 of a gallon We still colloquially call 750ml liquor bottles a “fifth”, so this is a plausible explanation. (Curiously, we never say “a

> 750ml is roughly 1/5 of a gallon We still colloquially call 750ml liquor bottles a “fifth”, so this is a plausible explanation. (Curiously, we never say “a fifth of wine” even though wine is sold in 750ml bottles too.)