鬼知道。（Guǐ zhīdào.）God knows.

Explanation

这句话的意思是:没有人知道或者没有人能说得清楚事情会怎么样或者事情的原因是什么。常在别人问自己问题而自己也不知道答案的时候使用，意思是“我也不知道”“我怎么会知道”。人们在用这句话的时候常带有一点儿恼怒或无可奈何的语气。也可以说“鬼才知道”，语气更强烈。有时候人们也说“天知道”，是一样的意思。
This sentence means that nobody knows the reason or can tell how the thing will be. It’s mostly used when you cannot give the answer to the question asked. It means “I don’t know it either” or “How can I know”. People usually use the sentence with a tone of anger or helplessness. You can also use “鬼才知道” to express a even stronger emotion. Sometimes people also say “天知道”, which has the same meaning.

Words:

雨yǔ:rain;
工作ɡōnɡzuò:work.

Sentences

1、 Jiă: Zhè yǔ yào xià dao shénme shíhou cái néng tíng a?
甲：这雨要下到什么时候才能停啊？
A: When will the rain finally stop?

Yǐ: Guǐ zhīdào a, zài xià jiù huí bu liǎo jiā le.
乙：鬼知道啊，再下就回不了家了。
B: God knows. We cannot go home if it continues like this.

2、Jiă: Dōu bā diǎn le, nǐ dàodǐ shénme shíhou cái néng bǎ gōngzuò zuò wán?
甲：都八点了，你到底什么时候才能把工作做完？
A: It’s already 8:00 p.m. When on earth can you finish the work?

Yǐ: Guǐ zhīdào, gěi wǒ bùzhì nàme duō huó, wǒ yí gè rén nǎ gàn de wán a!
乙：鬼知道，给我布置那么多活，我一个人哪干得完啊！
B: Don’t ask me! How am I supposed to finish all these assignments on my own?

原文链接