Genesis 38:6

She conceived, and bore a son; and he named him Er.

She conceived again, and bore a son; and she named him Onan.

She yet again bore a son, and named him Shelah. He was at Chezib when she bore him.

Er, Judah’s firstborn, was wicked in the LORD’s sight. So the LORD killed him.

Judah said to Onan, “Go in to your brother’s wife, and perform the duty of a husband’s brother to her, and raise up offspring for your brother.”

Onan knew that the offspring wouldn’t be his; and when he went in to his brother’s wife, he spilled his semen on the ground, lest he should give offspring to his brother.