Sarah "Granny" Aching (née Grizzel) was Tiffany Aching's grandmother, and very good friends with the Chalk Hill clan of the Nac Mac Feegle. To them she was the 'hag-o-the-hills' (that is, the 'witch of the Chalk'). She died two-years before the events in The Wee Free Men occurs, and, as such, only appears in flashbacks. There is substantial evidence that she was a witch, although she never said she was one. The Chalk's late Kelda confirms that Granny Aching was one, identifying that Granny Aching had "First Sight" & "Second Thoughts", as does her grand daughter and successor, Tiffany. When Tiffany speaks of Granny Aching to other witches, they often comment that she has many of the qualities of a very skilled witch, and one worthy of much respect, as her influence was such that the people she took care of ended up mostly taking care of each other (a feat only the most skilled witches are able to accomplish), even the formidable Granny Weatherwax commented that she would have liked to have met Granny Aching.
She could be described as 'salt of the earth' as well as the magma that runs beneath it. She was very important in the minds of the people of the Chalk, to the point where they called the thunder "Granny Aching cussin'", the vultures "Granny Aching's chickens", the fluffy little white clouds of summer "Granny Aching's little lambs" and said she cussed the sky blue. Although people laughed when they said these things, part of them was not joking. For every inhabitant of the Chalk, Granny Aching was the Chalk; its best shepherd, its wisest woman and its memory, to the point that even the Chalk's Feegles say of her that she "[told] the hills what they are, every day. She [held] them in her bones. She [held] 'em in her heart." Granny Aching was "as if the green downland had a soul that walked about in old boots and a smocking apron and smoked a foul pipe and dosed sheep with turpentine". She smoked Jolly Sailor tobacco, and had two sheepdogs—"Thunder and Lightning".
In The Wee Free Men, one of the last things that Tiffany saw when she was "truly awake" was Granny Aching, dressed as an ornamental shepherdess. Tiffany also became aware of people leaving small tributes at Granny's general gravesite, leaving her to wonder if Granny Aching is slowly becoming an actual cultural deity on the Chalk.
She was said to have a ringing personality, never lost a sheep in her life, and was taciturn 'unless something was worth saying'. Her petname for Tiffany, her favorite grandchild, was "Jiggit" (which means "Twenty" in the old counting language of shepherds, as Tiffany was her twentieth grandchild).