vigor: the productive strength held in a body, above all a man's prime, and by extension the property that strength piles up
Hebrew has several words for strength, and this one stays close to the body. It names the force a healthy creature can put out, the vigor stored in muscle and sinew. What sets it apart from the ordinary word for strength (כֹּחַ) is that Hebrew also hears in it a man's power to father children.
That is why the firstborn son carries such a striking title. He is never called the strongest of the sons; he is called the first yield of his father's vigor, the first thing produced by a man in his prime. When the Psalms describe the night Egypt's firstborn died, they reach for the very same phrase, so the blow falls not only on a generation of children but on the nation's power to keep producing them.
From vigor the word slides naturally into what vigor produces. A person who works hard ends up with something to show for it, so the same word can name his property. Ephraim boasts in Hosea that he has grown rich and "found" this for himself, and in Job a swindler's own hands have to give it back. English needs at least two words, "strength" and "wealth", for what Hebrew keeps in one.
Isaiah 40 turns the word into a quiet argument. The stars are marshaled by a God of immeasurable vigor, and a few lines later the same word names exactly what worn out human beings do not have. What runs dry in us never runs dry in him.