8 Then Judah said to Onan, “Sleep with your brother’s wife and fulfill your duty to her as a brother-in-law to raise up offspring for your brother.” 9 But Onan knew that the child would not be his; so whenever he slept with his brother’s wife, he spilled his semen on the ground to keep from providing offspring for his brother. 10 What he did was wicked in the Lord’s sight; so the Lord put him to death also.

– Genesis 38:8-10 (NIV)

Biblical Summary
Onan is a minor biblical person in the Book of Genesis chapter 38, who was the second son of Judah. Like his older brother Er, Onan was slain by God. Onan’s death was retribution for being “evil in the sight of the Lord” and disobeying a direct order from the Lord by being unwilling to father a child by his widowed sister-in-law.

Biblical Account
After Onan’s brother Er was slain by God, his father Judah told him to fulfill his duty to his brother by entering into a levirate marriage with his brother’s widow Tamar to give her offspring. Religion professor Tikva Frymer-Kensky has pointed out the economic repercussions of a levirate marriage: any son born to Tamar would be deemed the heir of the deceased Er, and able to claim the firstborn’s double share of inheritance. However, if Er were childless, or only had daughters, Onan would have inherited as the oldest surviving son.

When Onan had sex with Tamar, he withdrew before his orgasm and “spilled his seed [or semen] on the ground”, since any child born would not legally be considered his heir. The next statement in the Bible says that Onan did evil and that God slew him.