Heartless describes a person, an act, or even a rule that is cruel and shows no pity. The heart in this word is not the organ; English often uses heart for kindness, so heartless is ‘without that kindness.’ It is a strong, blaming word: a heartless remark, a heartless landlord. People also stretch it to systems — a heartless policy — which does not mean the policy has feelings; it means nobody made room for pity. Though actually, a long time ago it could also mean without courage, tired and with no fight left. That older meaning has almost gone. Today if you call someone heartless, you are saying they do not care that others are hurt, not that they are afraid. It is not a light joke-word unless the group already knows you are teasing.