Genesis 38:8

Judah said unto Onan. According to custom, Onan, as brother-in-law of Tamar, should have married the childless widow of his deceased brother and raised up a family for him. Onan, however, was loath to accept the responsibilities this involved, since the first-born son would not be his own but would perpetuate the family of the deceased and receive his inheritance. Onan’s conduct betrayed a lack of natural affection for his brother and a covetousness for his possessions and inheritance. Even worse, his conduct was an offense against the divine institution of marriage. This is a sad commentary on the low estate to which Jacob’s sons had fallen.

The custom of levirate marriage (from the Latin levir, “brother-in-law”), first mentioned here in the Bible, also existed, in varying forms, among other nations of antiquity, such as the Hittites. It was incorporated into the Mosaic legislation, with the provision that a brother-in-law might refuse to perform the duty. Such a refusal, however, was considered shameful, as the ceremony to be carried out in that case shows (Deut. 25:5-10). Ruth 4:5-8 records an example of such a refusal.