Green’s Dictionary of Slang

1. a notable eater of mutton.

1609		Man in the Moone 31: A sepulchre to seafish and others in ponds, moates, and rivers; a sharpe sheep-biter, and a marvellous mutton-monger, a gosbelly glutton.
1640		The Wandering Jew 38: He is a terrible Sheep-biter; a horrible Mutton-monger; a Gorbelly-Glutton.
1664		C. Cotton Virgil Travestie (1765) Bk I 22: They eat up Mutton, Guts and all, / Yet scarce could satisfy their Hungers, / These Trojans were such Mutton-mongers.
1674		T. Duffet Empress of Morocco Act I: He’ll be so cross, who can abide him If we a Sheeps-head don’t provide him? He is such an errant Mutton-monger.

2. a sheep-stealer.

1652		Mercurius Democritus 8-16 Dec. 185: The Muttonmongers feasting themselves this Christmas with the Mutton they stole out of Rumly Marsh.
c.1698		B.E. Dict. Canting Crew n.p.: Mutton-monger [...] a Sheep-stealer.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].