The amount that a barrel can hold; the capacity of one barrel.
From barrel plus the suffix -ful (meaning 'the amount that fills'), a productive English suffix that creates nouns meaning quantities, as seen in cupful, handful, and spoonful.
A barrelful was an actual unit of measurement in trade and commerce—different industries defined their 'barrels' differently, so a barrelful of beer wasn't the same as a barrelful of oil, creating constant confusion that merchants had to navigate.
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