In Chapter 10, Piggy and Ralph use idioms and colloquialisms to fret about their inability to leave the island:
“I mean it,” whispered Piggy. “If we don’t get home soon we’ll be barmy.” “Round the bend.” “Bomb happy.” “Crackers.”
Just as with the British dialect in the novel, these Britishisms and boyish slang words emphasize the youthfulness and otherness of Ralph and Piggy. Despite their dire circumstances, these characters are still children who describe going insane as being "barmy." Additionally, they do not belong on this island, a fact so apparent as to come out in their foreign dialogue.
All of these idioms and colloquialisms mean "crazy," as a reader might have been able to guess from context clues. We're not sure where the idiom "round the bend" comes from, but there are tons of guesses, including that it is related to naval slang for knots, the secluded locations of Victorian asylums, the diver's disease known as "the bends," and a metaphor which compares someone walking out of sight to someone doing something unthinkable or socially unacceptable.
"Bomb happy" is an idiom from WWII, and it originally described someone who enters a state of nervous breakdown from undergoing repeated bombings. Breakdown often looked like ecstasy or senseless happiness, hence the phrase "bomb happy."