空空如也

From the Analects, Book 9 (《論語·子罕》):

子曰：「吾有知乎哉？無知也。有鄙夫問於我，空空如也，我叩其兩端而竭焉。」 [Classical Chinese, trad.]

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子曰：「吾有知乎哉？无知也。有鄙夫问于我，空空如也，我叩其两端而竭焉。」 [Classical Chinese, simp.]

From: The Analects of Confucius, c. 475 – 221 BCE, translated based on James Legge's version

Zǐyuē: “Wú yǒu zhī hū zāi? Wúzhī yě. Yǒu bǐfū wèn yú wǒ, kōngkōng rú yě, wǒ kòu qí liǎngduān ér jié yān.” [Pinyin]

The Master said, "Am I indeed possessed of knowledge? I am not knowing. But if a mean person, who appears quite empty-like, ask anything of me, I set it forth from one end to the other, and exhaust it."

Originally, reduplication of 空 with particles was native to Classical Chinese grammar. In vernacular or modern Chinese, the chengyu with its air of classicality is often used for humorous effects.

空空如也

1. empty; vacuous; having nothing

   他講課的時候，教室經常空空如也。 [MSC, trad.]
   他讲课的时候，教室经常空空如也。 [MSC, simp.]

   Tā jiǎngkè de shíhòu, jiàoshì jīngcháng kōngkōngrúyě. [Pinyin]

   When he teaches, the classroom is often empty.