expression, godan verb (-su)
make a cutting remark; stab with a short blade
Idiomatic expression; the literal meaning is 'to stab someone with a short blade', but it is almost always used figuratively for delivering a sharp, concise criticism or witty remark.
彼の寸鉄人を刺すような一言に、会場が静まり返った。
The room fell silent at his cutting remark.
'Suntetsu hito wo sasu' means to strike at someone's weak point with a short phrase.