1.  firkin(n = noun.quantity) - a British unit of capacity equal to 9 imperial gallons;
is a kind of british capacity unit, imperial capacity unit
is a part of kilderkin
has parts: congius, gallon, imperial gallon

2.  firkin(n = noun.artifact) - a small wooden keg;
is a kind of keg

1.  A varying measure of capacity, usually being the fourth part of a barrel; specifically, a measure equal to nine imperial gallons. [1913 Webster]

2.  A small wooden vessel or cask of indeterminate size, -- used for butter, lard, etc. [1913 Webster]

firkin, n.
1 a small cask for liquids, butter, fish, etc.
2 Brit. (as a measure) half a kilderkin (8 or 9 gallons).

Etymology:
ME ferdekyn, prob. f. MDu. vierdekijn (unrecorded) dimin. of vierde fourth

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