Literally, it means to take a course opposite the apparent motion of the sun viewed from the Northern Hemisphere. Widdershins is cognate with the German widersinnig, i.e., “against” + “sense”. The opposite of widdershins is deosil, or sunwise, meaning “clockwise”.
A surprising number of things on the Disc that appear to be made up fantasy are actually based on real world words, concepts, cultures, and especially bits of folklore. Sometimes messed with a bit, but oftentimes played completely straight. And it makes the jokes even better when you look it up and figure out what Pratchett was referencing when he wrote it. Some of it is quite clever.
It’s true that the Discworld is a world, and a mirror of worlds (i.e. ours).
This is a big part of why I love them so much. Hardly a week goes by that I don’t think of the “boots theory of socio-economic unfairness” from Men at Arms!
Hogswatch is fantasy Hogmanay if you squint enough, for example.
This is why the Annotated Pratchett File make for a good reading companion.
Not only that but most of them are played down not up when compared to real life. The discworld is a lot less weird than real life.