Essentially, ‘Widdershins’ is old British world meaning to move in a counter-clockwise direction, although ‘to move contrary to the movement of the sun’ might be a more appropriate definition.
I’ve encountered the word twice in the last few weeks. The first was in an old English story about the young Charlemagne, in which his sister runs widdershins around a church and is whisked off to fairyland. The second time was in Poul Anderson’s The Broken Sword, which I’m currently reading, where an elf dances widdershins around a troll as part of the process for creating a changeling.
It is notable that both uses involve the fey or fairyland. There seems to be a connection between the unnatural movement of widdershins and the other world. In fact, the movement seems to have been considered so unnatural, that there isn’t a clear opposite to the word. That is, there seems to be no equivalent word for moving clockwise. Apparently moving around something was only worthy of mention if it was done ‘otherworldly’.