呦 is an interjection used at the start of a sentence to express surprise, dismay, or sudden realization — similar to 'Oh!' or 'Oops!' in English. It often carries a tone of mild alarm or unexpectedness. The reduplicated form 呦呦 (yōu yōu) appears in classical poetry as an onomatopoeia for deer calls, but in modern Chinese 呦 is most common as a standalone exclamation.