All through the Discworld series (well, the 'Witches' story arc, at least), whenever Margat refers to her former mentor, Goodie Whemper, Granny Weatherwax and Nanny Ogg chorus "may-she-rest-in-peace".
For example, in Lords and Ladies,
"I don't know what it means," said Magrat. "I mean, old Goodie Whemper-"
"-maysherestinpeace-" the older witches chorused.
"-told me once that the circles were dangerous, but she never said anything about why."
To me this seems as if it's one of the obscure references that Terry Pratchett peppers his books with, such as Granny Weatherwax continually munging the 'a man walked into a bar and asked for a alligator sandwich' joke right the way through Witches Abroad.
However, this occurs across so many books that I'm wondering if it's something more than just an obscure reference.
Why did Goodie Whemper have to rest in peace?