David is afraid of his grandmother in Indiana, and for good reason. There's no two ways about it: The woman is pretty mean. In an interesting twist on the communication troubles that course through David's life, whereas his parents pretty much don't talk to him about anything, it's the way Grandma Murphy speaks that causes some of their troubles together.
The way she talks suggests that she doesn't have much formal education. So, for instance, when David asks for one of the green beans she's snapping, she says:
"Summa them beansis tuffins." (1.328)
David doesn't understand her, though, so Papa John has to explain, "She said some of those beans might be hard to chew" (1.329). Later, when Grandma Murphy uses the word ain't, though, he finds himself with more trouble than just not understanding her. David makes the mistake of telling his grandmother, "Mama says people who say 'ain't' are stupid" (1.383), to which his grandmother responds with, "You all must think I'm real stupid, then" (1.386). Oops.
Things only go downhill from there, culminating in Grandma Murphy scalding David's hands in the sink in response to his refusal to go to bed early. When David asks his mom never to leave him alone with his grandma again, though, suggesting she's crazy, he's just told never to use that word again. The apple doesn't fall too far from the tree, it seems, when it comes to David's mother and grandmother.
Toward the end of the book, Grandma Murphy sets the house on fire with Papa John inside. Fortunately he lives, but she gets locked up in a mental institution for life. David's final dream is that his mother invites him inside the same institution, but he refuses to follow—he doesn't want to be part of the same lineage of unhappiness and abuse as his mother and her mother. Can't say we blame him.