What is the etymology of 赤の他人?

Question

彼は赤の他人だよ — He's a total stranger to me

How did "red stranger" come to mean "total stranger" in Japanese? Is there anything that makes this expression make sense more than "That's just what it means"?

Answer 1

The color term 赤 'red' is historically related to 明るい 'bright, clear' or 明らか 'evident'. From this, the 赤 in the expressions such as 赤の他人, 真っ赤な嘘, or 赤っ恥 means 'clear, complete'. This will make them mean 'complete stranger', 'complete lie', 'complete shameness'.

Answer 2

Consulting Gogen-Allguide, the 赤 used has the meaning of "complete and total", "completely" and "obvious/evident" (明らかな).

There is also a theory it is taken from the Buddhist terms 閼伽{あか} and 阿伽{あか} meaning "offering clean water", from "is cold to touch like water" to also mean "a person who is cold to touch", furthermore "a person who has absolutely no relation" but it's unlikely that's the case as there are other words/expressions which use 赤 for emphasis.