Green’s Dictionary of Slang

1. a fat person; thus gundy-gut adj., fat.

c.1698		B.E. Dict. Canting Crew n.p.: Gundigutts, a fat pursy Fellow.
1709		N. Ward Compleat and Humorous Account of Remarkable Clubs (1756) 282: Those Gundy-Gut Matrons who deal in Fee Lane Tripes.
1712		J. Arbuthnot Hist. of John Bull 60: What’s that to you, Gundy-guts (quoth Peg).
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. 1725].
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.

2. in sing. as gundy gut, the stomach.

1772		Bridges Burlesque Homer (3rd edn) 162: Like stout militia men who put / Their guns against their gundy gut.
1797		Bridges Burlesque Homer (4th edn) II 292: Slice after slice you’ll see him cut, / And stuff within his gundy gut.