From WordNet (r) 2.0 (August 2003) [wn]:
firkin
n 1: a British unit of capacity equal to 9 imperial gallons
2: a small wooden keg
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Firkin \Fir"kin\, n. [From AS. fe['o]wer four (or an allied
word, perh. Dutch or Danish) + -kin. See Four.]
1. A varying measure of capacity, usually being the fourth
part of a barrel; specifically, a measure equal to nine
imperial gallons. [Eng.]
[1913 Webster]
2. A small wooden vessel or cask of indeterminate size, --
used for butter, lard, etc. [U.S.]
[1913 Webster]