Green’s Dictionary of Slang

1. a very plain, fat, coarse woman.

1678		Behn Sir Patient Fancy V i: You’re a Dutch Butter-ferkin, a Kilderkin, a Double Jug.
c.1698		B.E. Dict. Canting Crew n.p.: Firkin of foul Stuff a very Homely coarse corpulent Woman.

2. a greedy, food-obsessed individual.

1771

R. King New London Spy 62: He is so taken up with the passion of gormandizing [...] and may be considered as a firkin of foul stuff, a lump of filth, or walking dunghill .