Definition of "firkin"

firkin

/ˈfɝkɪn/

noun
plural firkins
1. (Britain) A varying measure of capacity, usually being a quarter of a barrel; specifically, a measure equal to nine imperial gallons.
  Quotations
  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.
  1882, James Edwin Thorold Rogers, A History of Agriculture and Prices in England, volume 4, page 205
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.