firkin
/ˈfɜːkɪn/noun
1- chiefly historical a small cask used chiefly for liquids, butter, or fish〈主史〉（装液体、黄油或鱼等用的）小木桶。
1.1
- a unit of liquid volume equal to half a kilderkin(usually 9 imperial gallons or about 41 litres)费尔金, 小桶（液体容量单位, 等于½基尔德坎, 通常为9英制加仑或约41升）。
词源
Middle English ferdekyn, probably from the Middle Dutch diminutive of vierde 'fourth' (a firkin originally contained a quarter of a barrel).