- 寸(촌) inch
- 鐵(철) iron; steel
- 殺(살) kill
- 人(인) person
The 鐵(철) character refers to iron weapons. Here 살인(殺人) refers to killing a person's earthly desires, not to actually killing a person. The expression implies that a person can come to understand some truth from a simple lesson. Originally, the expression was referring to the truth of Zen Buddhism.
The expression is supposed to have originated in China during the Southern Song Dynasty (1127-1279) when a scholar by the name of 나대경(羅大經) was discussing Zen Buddhism with guests one evening. 나대경 is supposed to have said the following:
어떤 사람이 무기를 한 수레 가득 싣고 왔다고 해서 살인을 할 수 있는 것이 아니다. 나는 오히려 한 치도 안 되는 칼만 있어도 사람을 죽일 수 있다.
A person cannot kill someone by saying he brought along a wagon full of weapons. On the contrary, I can kill someone with just a knife that is not even one inch long.
You can read the Korean here.