Origin and meaning of the proverb 死人【しにん】に 口【くち】 無し【なし】

Question

The proverb 死人【しにん】に 口【くち】 無し【なし】 (the dead do not have a mouth) is listed e.g. here and I've also heard it from my (native) teacher.

Here's what I'm wondering:

1. From what I've heard so far, it should be understood to mean "You can say anything about a dead person. They will not talk back". Is that how everyone will understand it? I want to be sure that I'm using it correctly. An alternative interpretation would be "If you want someone to keep her mouth shut, kill her". Can I be sure that this is never meant? (I could imagine such a thing being said in a gangster movie)

2. How old is this saying/are its origins known? Is it connected to a religion, is it maybe originally Chinese, etc.?

Answer 1

From what I gather, the connotation of the English phrase "Dead men tell no tales" is the latter -- "It's safe to kill everyone who knows the secret". Am I right?

That's not the primary meaning of Japanese 死人に口なし, although it looks very similar. As your teacher said, this should be understood as "You can easily accuse a dead person, even unjustly, because they can't argue back". This is usually used for someone who is already dead, and that person does not necessarily have a secret.

死者は何も語れないので、何の証言も釈明もできない。口のきけない死者は生きた者の思うように利用されやすいということ。(故事ことわざ辞典)

死人は無実の罪を着せられても釈明することができない。また、死人を証人に立てようとしても不可能である。 (デジタル大辞泉)

And I think this proverb is frequently used in the sense along the lines of "If you want to ask something from a dead person, it's too late", typically in police dramas.

That said, if Japanese people see 死人に口なし in subtitles of foreign spy films, etc., I believe they can easily understand that implication. ("Ah, this must be a euphemistic and cool way of saying kill'em!")

Answer 2

Additional information on question #2, at OP's request:

日本国語大辞典 records a usage that dates back to 1788, which is mid-Edo period.

雑俳・柳多留‐二二〔１７８８〕「死人に口なし置みやげとぬかし」

And its relationship with Chinese is unclear. They have a bit different expression 死无对证 "the dead cannot testify" in Chinese, which seemingly traces back to Yuan dynasty. Yet it seems several languages also have similar sayings.