By the life of Pharaoh, as sure as Pharaoh lives. It seems to be the form of an oath in use among the Egyptians, as afterwards the Romans used to swear by the name, genius, health, and life of their emperors. Compare 1 Samuel 1:26, 1 Samuel 17:55 2 Kings 2:2 Ezekiel 33:11. And it is not strange that Joseph through human infirmity was carried by the stream of the general practice of the court, especially when the law of God was not yet delivered concerning the appropriation of oaths unto God.
Related Commentaries of Genesis 42:15
Genesis 42:15
15 Hereby ye shall be proved: By the life of Pharaoh ye shall not go forth hence, except your youngest brother come hither.