“Batty” means crazy or outlandish. The boys use the word to dismiss ideas they don’t understand or don’t want to take seriously. When talk of the “beast” starts to spread, for example, more rational characters like Piggy react by calling such fears ridiculous or irrational—essentially “batty”—because they don’t believe in a real monster.
What matters is how the word works socially. Calling something “batty” becomes a way to shut down fear or uncomfortable truths. When Simon suggests that “maybe it’s only us”—that the beast is actually inside the boys—his idea sounds strange to the others, and they treat it like nonsense rather than face what he’s implying. So, “batty” reflects the boys’ instinct to reject ideas that challenge their sense of control or force them to confront their own darker impulses. That habit of dismissing what feels unsettling instead of examining it feeds the larger collapse of reason on the island.