firkin

noun

1. (British)A varying measure of capacity, usually being a quarter of a barrel; specifically, a measure equal to nine imperial gallons.

   “23 Hen. VIII, cap. 4... The barrel of beer is to hold 36 gallons, the kilderkin 18 gallons the firkin 9. But the barrel, kilderkin, and firkin of ale are to contain 32, 16, and 8 gallons.”

   “But for his other guests he let bear in the massy cups of silver, and the great eared wine jars holding two firkins apiece, and he let pour forth to the Witches and the Foliots, and they drank the cup of memory unto King Gorice XI., slain that day by the hand of Goldry Bluszco.”

   “Barrels came in firkins, nine gallons; kilderkins, eighteen gallons; halves, twenty-seven gallons; barrels, thirty-six gallons and hogsheads, fifty-four.”

2. (US)A small wooden vessel or cask of indeterminate size, used for butter, lard, etc.
3. A measurement for the mass of butter, equalling 56 pounds.