It means "someone who is cold-blooded". If you say 그는 칼로 찔러도 피 한방울 나지 않겠다. you mean, "that guy looks cold-blooded" kinda lack of pity or sympathy

I'd venture a guess and say "Even if you/he/she/I stab/s him/her/it, not a single drop of blood will come out."