etsem

This is the plain, physical sense and by far the most common one. A single bone, the full set of bones, or a person's skeleton after death all fall here.

Because the bones are what hold a body up and carry its strength, the Hebrew often locates a person's inner state in them. When someone is overwhelmed by fear, grief, sickness, or guilt, the Bible says it is felt in the bones: they shake, melt, waste away, burn, or dry up. The same picture works the other way: good news, healing, and a settled heart are said to refresh or strengthen the bones. None of this is a separate word; it is the one word for bone, used for the deepest layer of a person because that is where the body's structure and stamina sit.

Dead bones carry their own weight in the text. To leave a body unburied, to scatter or burn someone's bones, or to disturb a grave was a grievous dishonor, while gathering and carrying home a person's bones was an act of respect and faith. The valley of dry bones brought back to life is the most famous image built on this sense.

From naming the body's frame, the word came to stand for the body or the person as a whole, the very stuff someone is made of. When a man calls another person his bone, he is not talking about a skeleton; he is claiming shared substance.

This is clearest in the kinship phrase "my bone and my flesh," which Adam says of the woman taken from him and which families and tribes use to claim each other as their own kin. The point is closeness and common origin: you are made of what I am made of. Here the word means something like one's own self or one's own flesh and blood, and an English translation has to reach for "self," "body," or "flesh and blood" rather than "bone."

In a fixed expression the word stops meaning bone at all and works like the English words "very" or "selfsame." "In the bone of this day" means "on this very day," pinning an event to the exact day and no other. The idea seems to have grown out of bone as the hard core or substance of a thing: the bone of the day is its solid center, the day itself.

The same instinct shows up when the word names the core or essence of something else, such as a sky described as clear in its very substance. A reader needs a different English word here, because nothing about bones is being said; the word is being used to say "this exact one."