The answer that seems to be most common online is that the name probably came from the sound the pudding made as it bubbled and grunted beneath the biscuitlike topping. But one of our Mennonite readers told me this: I think that “grunt” in that recipe name is actually a PA Dutch word meaning something like earth or soil. The topping in the recipe resembles earth covering the apples. In Dutch I would say, “I wash the “grunt” off the freshly dug potatoes, before bringing them in.” So that, to this ear, seems like a much more plausible answer than bubbling pudding!