‘Widdershins’ is in fact an existing word meaning ‘counter-sunwise’, i.e. counter-clockwise in the Northern hemisphere, clockwise down South. A synonym for ‘turnwise’ is deosil, which helps explain Ankh-Morpork’s Deosil Gate as found on the The Streets of Ankh-Morpork Mappe.
Widdershins is also the name of the planet where Dom, the hero from The Dark Side of the Sun lives.