By the life of Pharaoh— This was a solemn protestation; as he valued the life and honour of his prince, he would do so and so. Judah explains it in this manner to his father, ch. Genesis 43:3. The man did solemnly protest unto us; and certainly Judah, who heard him, must best understand his meaning. Yet the protestation is not to be vindicated in Joseph. There can be no doubt, that it was customary among the heathens to swear, properly speaking, by the life or health of their kings and emperors, as the Jews did by their living God. Joseph used this phrase, the better to conceal himself from his brethren, and to make them think him an AEgyptian.
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.