What does 촌철살인(寸鐵殺人) mean?

촌철살인(寸鐵殺人) literally means "kill someone with inch-long steel," but it implies that it is easier to catch an opponent off guard with one word, than with a thousand. Here are the characters:

• 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.