John Gill Commentary Genesis 38:8

"And Judah said unto Onan, Go in unto thy brother`s wife, and perform the duty of a husband`s brother unto her, and raise up seed to thy brother." — Genesis 38:8 (ASV)

And Judah said to Onan
Some time after his brother's death: go in to your brother's wife, and marry her; Moses here uses a word not common for marriage, but which was peculiar to the marrying of a brother's wife according to a law given in his time: it appears to have been a custom before, and which the patriarch might be directed to by the Lord, in such a case when a brother died, and left no issue, for the sake of multiplication of seed, according to the divine promise, and which in the time of Moses passed into a law, see (Deuteronomy 25:5);

and raise up seed to your brother; that might bear his name, and enjoy the inheritance. For this law or custom was partly political, to continue the paternal inheritance in the family, and partly typical, to direct to Christ the firstborn among many brethren, (Romans 8:29) , who in all things was to have the preeminence, (Colossians 1:18);

and this was not taken from the Canaanites, among whom Judah now was, but from the ancient patriarchs, which they no doubt had from divine revelation, and was taught in the school of Shem, and handed down from father to son; for as to this being a law among the Egyptians in later times, and which continued to the days of Zeno Augustus ^F17 , it is most likely they took it from the Jews.

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FOOTNOTES:

• F17: Justinian. Cod l. 5. tit. 6. leg. 8.