the firstborn: the first child or animal a mother bears, and the rank and rights that come with being first
At its core this word names whoever or whatever is born first, and almost always it carries the weight of what being first meant in Israel. The eldest son inherited a double share and led the family after his father, so calling someone the firstborn says far more than the order they happened to arrive in. It marks them as the one with first claim and first honor.
The same word covers the first young of cattle, sheep, and goats, which God claimed as his own and which had to be redeemed or offered. The household firstborn and the animal firstling sit side by side in the law for this reason: both are the opening of the womb, and both belong, in a special way, to God.
Because being first carried such honor, the word stretches naturally into a title of rank. To call someone the firstborn can mean the foremost, the one set above all others, even where no birth order is in view. Israel is God's firstborn among the nations, and the king is made firstborn, the highest of the kings of the earth. The reverse image works too: the firstborn of death and the firstborn of the poor name the most extreme or the most wretched of their kind.