Genesis 38:9

9 But Onan knew that the descendant wouldn't belong to him, so whenever he slept with his brother's widow, he wasted his semen on the ground to avoid giving his brother a descendant.

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 Er angered the LORD. So the LORD took away his life.

8 Then Judah said to Onan, "Go sleep with your brother's widow. Do your duty for her as a brother-in-law, and produce a descendant for your brother."

9 But Onan knew that the descendant wouldn't belong to him, so whenever he slept with his brother's widow, he wasted his semen on the ground to avoid giving his brother a descendant.

10 What Onan did angered the LORD so much that the LORD took away Onan's life too.

11 Then Judah said to his daughter-in-law Tamar, "Return to your father's home. Live as a widow until my son Shelah grows up." He thought that this son, too, might die like his brothers. So Tamar went to live in her father's home.