The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

firkin
    n 1: a British unit of capacity equal to 9 imperial gallons
    2: a small wooden keg

Easton's 1897 Bible Dictionary:

Firkin
   Used only in John 2:6; the Attic amphora, equivalent to the
   Hebrew bath (q.v.), a measure for liquids containing about 8 7/8
   gallons.


Bouvier's Law Dictionary, Revised 6th Ed (1856):

FIRKIN. A measure of capacity equal to nine gallons. The word firkin is also
used to designate a weight, used for butter and cheese, of fifty-six pounds
avoirdupois.