Onan is the second son of Judah, whose story is recounted in Genesis 38. Onan’s older brother, Er, marries a woman named Tamar. However, Er is wicked and put to death (v. 7), leaving Tamar a widow. Judah instructs Onan to marry Tamar and produce a child, thereby continuing the line of Er (i.e. levirate marriage; see also the law in Deuteronomy 25:5-10). Onan sabotages this responsibility (v. 9), and he is put to death as well.