The noun FIRKIN has 2 senses: 1. a British unit of capacity equal to 9 imperial gallons. 2. a small wooden keg. Familiarity information: FIRKIN used as a noun is rare.ypernyms (firkin is a kind of...): British capacity unit; Imperial capacity unit (a unit of measure for capacity officially adopted in the British Imperial System; British units are both dry and wet). Meronyms (parts of firkin):