to be or become hard: too difficult to manage, harsh or heavy in force, or (of the heart and neck) stubbornly unwilling to yield
The one picture underneath every use of this word is hardness. Something can be hard the way a task is hard, too difficult to handle. It can be hard the way a blow or a burden is hard, harsh and heavy on the people who feel it. And it can be hard the way a person becomes hard, stiffening against every appeal until nothing can move them. The word runs across all three without changing its core idea; only what is being called hard changes.
The great majority of its appearances are about people, and they are almost always negative. To make one's neck or heart hard is to set oneself against God: to hear the command or the warning and refuse to bend. This is the language the Bible uses for Pharaoh, and again and again for Israel, the people repeatedly called stiff-necked. The image is physical, a neck that will not turn and an unyielding heart, but the meaning is moral, a settled refusal to obey or repent.
A smaller cluster keeps the plain, physical sense. A legal case can be too hard to decide, a request too hard to grant, an older group's words harsher than another's, a hand pressing down hard on a city, a king's yoke made heavier than before, or a mother's labor that turns hard and costs her life. In these the word simply measures difficulty or severity, with none of the note of rebellion that dominates the rest.