A son of Judah who was condemned for failing in the duty of a brother. When his brother died, Onan wilfully disobeyed the Levirate law (Deut. 25: 5–10) which required him to make good his deceased brother's lack of offspring (Gen. 38: 6–10), by practising coitus interruptus (?), and, according to the narrative, was punished by death (38: 10).