firkin

n. small barrel; unit of capacity equal to approximately 9 gallons or 34 liters (British)

A Firkin is an old English unit of volume. The name is derived from the Middle Dutch word vierdekijn, which means fourth, i.e. a fourth of a full-size barrel. The firkin (a firkin of water) is the base unit of mass in the FFF (Furlong/Firkin/Fortnight) System of units.Nor need you mind the serial ordeal Of being watched from forty cellar holes As if by eye pairs out of forty firkins. —  Robert Frost, "Directive" For beer and ale a firkin is equal to 9 Imperial gallons (about 40.915 l) or a quarter of a barrel. Casks in this size (themselves called firkins) are the most common container for cask ale. The word "firkin" (as in "Fox & Firkin") is frequently considered a suitably atmospheric word by those naming an English-style pub — by implication, the establishment will thus be either a new pub in the UK or a foreign imitation of a British pub.
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Noun
1. a British unit of capacity equal to 9 imperial gallons
(hypernym) British capacity unit, Imperial capacity unit
(part-holonym) kilderkin
(part-meronym) gallon, Imperial gallon, congius
2. a small wooden keg
(hypernym) keg

n. firkin, small barrel; unit of capacity equal to approximately 9 gallons or 34 liters (British)

See: Weights And Measures AND Measures