Liquid measurement.

And there were set there six waterpots of stone, after the manner of the purifying of the Jews, containing two or three FIRKINS apiece. Jesus saith unto them, Fill the waterpots with water. And they filled them up to the brim. ( John 2:6-7 )

Source: A King James Dictionary. (Used with permission. Copyright © Philip P. Kapusta)

Bibliography Information

"Entry for 'Firkin'". A King James Dictionary.