From 《風賦》allegedly composed by Song Yu:

枳句來巢，空穴來風。

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枳句来巢，空穴来风。

From: Song Yu, 《風賦》

Zhǐ gōu lái cháo, kōng xué lái fēng.

The twisted branches of a trifoliate orange tree invite birds to nest, and hollows and cracks summon the wind.

Also see the usage notes below.

空穴來風

1. (of a rumour, literary, dated) to be not completely unsound; to have a basis for its existence
2. (of a rumour, in modern uses, sometimes proscribed) to be completely unsound; to be utterly baseless

This term can both mean "to be not completely unsound" and "to be completely unsound", with different dictionaries offering competing (and contradictory) definitions of the term, although in modern usage the latter definition appears more frequently. However, this latter definition is sometimes proscribed based on the etymology and historical usage of the term.

It is agreed that the modern usage arose from a reinterpretation (often framed as a mis-interpretation) of the original idiom. According to Lǐ (2013), the reinterpretation likely hinges on the polysemy of 空 (kōng): For the definition of "to be not completely unsound", 空 means "empty", i.e. that an empty hole easily attracts wind, implying that a rumour has reason to have arisen. For the definition of "to be completely unsound", 空 is interpreted as "none", i.e. that no hole exists which could have given rise to wind, implying that a rumour has arisen from nothing. Lǐ (2013) contends that 空穴來風 should not be viewed as a single lemma but as two separate idioms.

From its 6th edition, 《現代漢語詞典》 (Dictionary of Contemporary Chinese) records both meanings under its entry for 空穴來風. Prior editions had only recorded the definition "to be completely unsound".