How does Walter Cunningham claim he almost died his first year of school?

At the beginning of Chapter 3, Scout is beating up Walter Cunningham when Jem intervenes and breaks up the fight. Jem invites Walter Cunningham to his house for a meal, and Walter reluctantly accepts. As the children walk to the Finch residence, Jem references Boo Radley by saying, "A hain't lives there." (Lee 31) Walter tells Jem that he almost died his first year of school after eating Radley's "poisonous pecans." Walter elaborates and tells Jem that Boo purposely poisons the pecans and puts them over on the school side of the fence. This is another example of how rumors about Boo Radley circulate. The children imagine a terrifying creature whose top priority is to harm others. Throughout the novel, as Scout and Jem mature, they learn the true nature of Boo Radley, who is just a reclusive, caring neighbor.