Literally 'hardened heart' — having a heart tough enough to do something that would normally cause guilt or pity. Almost always used in negative or questioning contexts.
忍心 is almost always used in questions (怎么忍心?) or negatives (不忍心, 哪忍心). Saying 我忍心 (I have the heart to be cruel) would sound strange and cold. The word exists precisely to question or deny one's ability to be cruel.
