Genesis 38:9

9 And Aunan, knowing that the seed should not be his—it came to pass when he went in to his brother's wife, that he spilled upon the ground, so that he should not give seed to his brother's wife.

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, the first-born of Judas, was wicked before the Lord; and God killed him.

8 And Judas said to Aunan, Go in to thy brother's wife, and marry her as her brother-in-law, and raise up seed to thy brother.

9 And Aunan, knowing that the seed should not be his—it came to pass when he went in to his brother's wife, that he spilled upon the ground, so that he should not give seed to his brother's wife.

10 And his doing this appeared evil before God; and he slew him also.

11 And Judas said to Thamar, his daughter-in-law, Sit thou a widow in the house of thy father-in-law, until Selom my son be grown; for he said, lest he also die as his brethren; and Thamar departed, and sat in the house of her father.