Guild of Plumbers and Dunnikindivers
Guild motto: NON ANTE SEPTEM DIES PROXIMA, SQUIRI ...... Not before next week, Squire
In Pyramids, we learn about the Guild of Plumbers and Dunnikindivers and what the latter half of this guild does for a living. This is in the context of Teppic seeking to improve the plumbing and sanitation of Djelibeybi by importing experts from Ankh-Morpork. When the expected advisors from the Guild of Plumbers and Dunnikindivers fail to turn up, he blames Dios, suspecting the High Priest of seeking to maintain the status quo by detaining or otherwise diverting them. (It turns out they really have been taken by pirates, and Dios is on this occasion blameless.)
Simply put, a dunnikindiver dives in what Australians called dunnies. This is not for the sport of it: a dunnikin is a septic tank. These unappealing but necessary fixtures need to be built, maintained, and eventually emptied, and the dunnikindiver is the man who does it. (This can be not only a filthy job but a dangerous one: in the high summer of 1785, the Prefect of Paris ordered that some of the city's massive septic tanks should be emptied, as the smell was getting unbearably offensive. This was "offensive" by the standards of eighteenth century Europe. At least three dunnikindivers died of the fumes before the Prefect abandoned the project...)
Brother Dunnikin in Guards! Guards! is such a man, and his colleagues in the Elucidated Brethren of the Ebon Night should be glad that his only body odour is the comparatively mild perfume of halitosis. The rest, he is presumably aware of and can guard against, as elsewhere in the chronicles there is a mention that Dunnikindivers are men who, when off duty, smell only of soap and obsessive cleanliness. (Halitosis is the one that not even your best friends can bear to tell you about, so he is probably in blissful ignorance about this. Unlike the rest of the Brethren.)
Given the general primitive state of plumbing in Ankh-Morpork, where the river is viewed as convenient drain for all the city's effluent, this cannot be one of the richest Guilds in the city, and only outside contracts such as the one in Djelibeybi must keep the membership in work....
The guild motto is believed to refer to a kind of specialised time travel possessed by plumbers which effectively means they have no grasp of the present tense: "immediately" or "today" to a plumber means "I might turn up next week, most likely the week after"
The Guild trains apprentices to a high standard: a central tenet of training is "never, ever, promise to do today what may be put off till next week, ideally the week after"
The current president is Sir Charles Lavatory of Mollymog Street. His patented device cleans and flushes beautifully, but in keeping with the general miasma of Ankh-Morpork, not to anywhere in particular. The Lavatory device is fast replacing the earlier model patented by a previous Guild luminary, William de Privy.
However William Privy's invention seems not always to be plumbed in: mention is made of Nanny Ogg having to move her privy, which suggests it is an earth closet, rather than water closet.
 A squire was the local landowner (gentry but not nobility) in England over the past few centuries, but has come to be used colloquially by a certain class of people so that they can address one without using the word "sir".