1. On the Discworld Widdershins is one of the four basic directions , and the opposite of turnwise. By going widdershins, you go against the turning direction of the Disc, in other words anti-clockwise.
Widdershins (sometimes withershins, widershins or widderschynnes) is a word which (usually) means counterclockwise. However, in certain circumstances it can be used to refer to a direction which is against the light, i.e. where you are unable to see your shadow. It is cognate with the German language widersinnig, i.e., "against" + "sense". The term "widdershins" was especially common in Lowland Scots, and was known in Scottish Gaelic as tuathal, which uses the same root as tuath meaning "north", the opposite of widdershins is deiseil or sunwise. In the southern hemisphere, the sun goes anti-clockwise, but in the northern hemisphere, it goes clockwise, which is where the term "sunwise" originates from. Because the sun played a highly important role in primitive religion, to go against it was considered very bad luck for sun-venerating traditions.
It was considered unlucky in former times in Britain to travel in an anticlockwise (because anti sun wise) direction around a church and a number of folk myths make reference to this superstition, e.g. Childe Rowland, where the protagonist and his sister are transported to Elfland after his sister runs widdershins round a church. There is also a reference to this in Dorothy Sayers's novels The Nine Tailors and Clouds of Witness ("True, O King, and as this isn't a church, there's no harm in going round it widdershins").