天晓得 is a colloquial interjection meaning 'Heaven knows' or 'Who knows?' — it expresses that the speaker has no idea about something or finds it hard to understand, often with a tone of helplessness or exasperation. Use it at the beginning of a sentence, like 天晓得他为什么不来. Unlike English 'God knows', it has no religious overtone in Chinese; it's just a common fixed phrase.