Nanny Ogg is a wonderful character. Have you read her description in the Discworld Companion?

It says, "After an adventurous girlhood -always chaste, as she says, and often caught - and a period as a maid at Lancre Castle, Gytha Ogg was accepted by Biddy Spective as her successor to the cottage in Lancre Town where she brought to the craft of witchery an honest, earthy outlook, a non-judgemental understanding of human nature and the ability to crack walnuts with her knees."!

And plenty more of course.