Answer:
She means that the story Grandaddy is going to say is a lie.
Step-by-step explanation:
In lines 8-9, when the author's grandmother says, "Hold it, let me leave the room, lightning's going to strike," she means that ______.
The metaphor can mean that someone is going to tell a lie, and in this case, it means that Grandaddy is going to tell the lie.