Genesis 38:9

9 But since Onan knew that the offspring would not be his, he spilled his semen on the ground whenever he went in to his brother's wife, so that he would not give offspring to his brother.

Genesis 38:9 Meaning and Commentary

Genesis 38:9

And Onan knew that the seed should not be his
Should not be called a son of his, but a son of his brother Er; this is to be understood only of the firstborn; all the rest of the children born afterwards were reckoned the children of the real parent of them; this shows this was a custom in use in those times, and well known, and was not a peculiar case: and it came to pass, when he went in unto his brother's wife;
to cohabit with her, as man and wife, he having married her according to his father's direction: that he spilled [it] on the ground, lest he should give his seed to
his brother:
lest his brother's wife he had married should conceive by him, and bear a son that should be called his brother's, and inherit his estate; and this is the sin, which from him is called Onania, a sin condemned by the light of nature, as well as by the word of God, and very prejudicial to mankind, as well as displeasing to God, as follows:

Genesis 38:9 In-Context

7 But Er, Judah's firstborn, was wicked in the sight of the Lord, and the Lord put him to death.

8 Then Judah said to Onan, "Go in to your brother's wife and perform the duty of a brother-in-law to her; raise up offspring for your brother."

9 But since Onan knew that the offspring would not be his, he spilled his semen on the ground whenever he went in to his brother's wife, so that he would not give offspring to his brother.

10 What he did was displeasing in the sight of the Lord, and he put him to death also.

11 Then Judah said to his daughter-in-law Tamar, "Remain a widow in your father's house until my son Shelah grows up"—for he feared that he too would die, like his brothers. So Tamar went to live in her father's house.