Genesis 38:9

9 But when Onan knew that the seed should not be his own, it came to pass when he went in to his brother's wife, that he spilled [it] on the ground, in order to give no seed 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 And Er, Judah's firstborn, was wicked in the sight of Jehovah, and Jehovah slew him.

8 Then Judah said to Onan, Go in to thy brother's wife, and fulfil to her the brother-in-law's duty, and raise up seed to thy brother.

9 But when Onan knew that the seed should not be his own, it came to pass when he went in to his brother's wife, that he spilled [it] on the ground, in order to give no seed to his brother.

10 And the thing which he did was evil in the sight of Jehovah, and he slew him also.

11 And Judah said to Tamar his daughter-in-law, Remain a widow in thy father's house, until Shelah my son is grown; for he said, Lest he die also, as his brethren. And Tamar went and remained in her father's house.