the breast of a sacrificial animal: the front cut presented to God and then kept by the priests as their portion
This is a butcher's word that the law turned into a legal one. It never describes a living body. All twelve of its verses stand inside the rules for sacrifice, where an animal has already been killed and its parts are being divided up, and the question the law is answering is who gets to eat which piece.
The answer is the priests. In the shared sacrifice the Bible calls the peace offering, where the worshipper eats part of the meat, the same few steps repeat every time this word appears:
- the fat is laid on top of the breast and carried to the altar to be burned
- the breast itself is raised and presented before the LORD
- what has been presented comes back to the priests, to be eaten by their families in a clean place
More often than not the breast travels in a fixed pair with the right thigh (שׁוֹק): the breast as the piece that is presented, the thigh as the piece that is handed over. Together they are the standing wage Israel owes the men who serve at the altar, and the pairing is so familiar that Numbers 18:18 can simply say the meat of a firstborn animal belongs to the priest "like the breast and like the right thigh" and trust the reader to know what that means.
Two verses sit slightly apart. At the ordination of Aaron the breast of the ram goes to Moses rather than to the priests, because Moses is the one serving at the altar that week. Nothing about the word changes there. It is the same cut, claimed by whoever is doing the priest's work.