Final answer:
Atticus claims that Mayella did something so terrible that it was unspeakable to society.
Step-by-step explanation:
According to the given text, Atticus claims that what Mayella did was so terrible that it was unspeakable to society. The specific action is not explicitly mentioned in the given text, but it suggests that Mayella committed a heinous act that has had a profound impact on the house and the surrounding environment. Atticus also mentions that if people believed in the unseen as they do in the seen, life would be different.