If someone is hard-hearted, they see that another person is hurting and they still do not care — or they will not let that pain change what they do. We use it about people who will not help, will not forgive, or who follow a rule even when it is cruel. It is colder than strict. You can be strict and still care; hard-hearted means the caring is missing, or looks missing. Stories use it for kings, villains, and anyone who seems to have no soft place inside. The word feels a little old and bookish. Young people in casual talk more often say cold or mean. It is not a doctor's word about the heart in your chest. Though actually, people also throw it in anger at someone who is only being firm — a parent who will not buy a toy — which is looser than the tidy picture.