Fornication is a time-honoured technical term whose origins are lost in the mists of time, but which is thought to have arisen as a means of giving an exact description to a practice associated with the world's oldest profession.
The fornication in the vaulted cellars of the Royal Bank of Ankh-Morpork has been described as absolutely wonderful, and so prolific that there is more of it down there than in the rest of Ankh-Morpork put together. There is no special time to turn up to see it, but it is always best to arrange an appointment first - just contact Mavolio Bent and he will see to it, as it is most convenient if small well-ordered groups visit in normal banking hours.
The noun and adjectival sense all apparently derive from the Latatian word fornix, meaning a non-circular, pointed, arch, usually brought together in pairs at right-angles to each other, to provide the strongest possible support for a stone-flagged floor on an upper floor, as well as to provide a soaring vista of rounded curves disappearing up to a shared point up in the roof-space.
Well, what did you think we were referring to?
It is quite possible that this specialised descriptive term from the practice of master masonry is not known to many people.... you may be surprised.
Further discussed in Making Money.
Coming down the centuries to us from William Shakespeare, we have the colourful demotic phrase to make the beast with two backs, denoting a particularly intimate, if not always private, human activity. What else is a fornicated arch - two arches brought together at a commmon apex, at right angles to each other, remember - except a kind of gestalt entity with two "backs"?