John Stow published his Survey of London in 1562. Under the heading Statutes of the Streets of this City is this: “No Goungfermour shall carry any Ordure till after nine of the Clocke in the night.” That identifies the goungfermour as one of the lowest orders of men, a dung carrier, nightman or cleaner-out of privies, who dealt with the product that a squeamish later generation would refer to euphemistically as night soil.
As goungfermour, the word is known almost exclusively from the statute. It’s a variation on a term that appears much more often as gong farmer or gang farmour.
The Oxford English Dictionary suggests that farmer here meant someone who cleanses or purifies a place. It has been suggested that this is from an old verb fay, to cleanse, from which came the noun faymer, one who cleanses, which changed to farmer by imitation. The Oxford English Dictionary argues, however, that it’s from a different verb, the Old English feormian, of similar sense.
The first part, gong, is an Old English word we might replace with privy, jakes, latrine, loo or other related term. Gong is from gang, one’s walk or gait (a sense that survives in German and related languages), so a gong was a place where one “went” to do what was necessary.
Gong farmer does still turn up from time to time, for example in discussions of the sanitary arrangements of castles. It also appears as an exoticism in historical or fantasy writing:
It seemed to me that nearly everyone in Hesperu, from the lowliest gong farmer to the King, was a slave of some sort.
Black Jade, by David Zindall, 2005.