a small barrel (= a round container) used in the past for liquids and butter, normally holding 9 imperial gallons or around 41 litres

He had a firkin of ale hidden in the barn. 他在谷仓里藏了一费金桶的啤酒。

That year his seven cows produced only two firkins of butter. 那一年，他的七头奶牛只产出了两费金桶的黄油。