Green’s Dictionary of Slang

tun of grease n.

a fat person.

1703		T. Baker Tunbridge Walks IV i: To see a Tun o’ Grease, with a broad fiery Face, and a little black Cap, waddle after a Bowl.
1723		‘Whipping-Tom’ Democritus III 39: She had more Reason to hide her Face than her B---ch; for the Face of this Female Tun of Fat, which wanted paving with Cherry-Stones, was broader than the full Moon.