What are you on about?

• What are you on about?: いったいなんの話をしてるんだ？

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• 'What are you on about?' said Ron, but Harry, sprinting across the grounds towards the Forest, didn't answer.
  （「なんの話だよ？」ロンがそう言ったが、ハリーは無言のまま校庭を突っ切り、森へと向かった。）
  J. K. Rawling, Harry Potter and the Philosopher's Stone, Bloomsbury, p. 193