expression, godan verb (-su)
make a cutting remark; kill with a short blade
Idiomatic expression. The literal meaning is 'to kill a person with a short blade', but it is almost always used figuratively to mean delivering a sharp, incisive remark that cuts deeply. The figurative sense is the primary modern usage.
彼の一言は寸鉄人を殺すような鋭さだった。
His single remark was as sharp as a short blade that kills.
寸鉄人を殺すとは、短い言葉で相手をやり込めることだ。
'Sun tetsu hito o korosu' means to overwhelm someone with a short, cutting phrase.