Genesis 38:9

9 And he knew that sons should not be born to him; and he entered [in] to the wife of his brother, and shedded his seed into the earth, lest the free children should be born by the name of the dead brother; (But Onan knew that any sons who would be born would not be his; so he slept with his brother's wife, but he poured out his seed onto the ground, so that no children would be born in the name of his dead 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, the first begotten son of Judah, was wayward in the sight of the Lord, and therefore he was slain of the Lord (and so the Lord killed him).

8 Therefore Judah said to Onan, his son, Enter thou [in] to the wife of thy brother, and be thou fellowshipped to her, that thou raise seed to thy brother. (And so Judah said to his son Onan, Sleep with thy brother's wife, and be fellowshipped to her, and so raise thou up seed, or some sons, for thy brother.)

9 And he knew that sons should not be born to him; and he entered [in] to the wife of his brother, and shedded his seed into the earth, lest the free children should be born by the name of the dead brother; (But Onan knew that any sons who would be born would not be his; so he slept with his brother's wife, but he poured out his seed onto the ground, so that no children would be born in the name of his dead brother;)

10 and therefore the Lord smote him (and so the Lord struck him down), for he did (an) abominable thing.

11 Wherefore Judah said to Tamar, his son's wife, Be thou a widow in the house of thy father, till Shelah my son waxed (until my son Shelah hath grown up); for he dreaded lest also he should die as his brethren (did). And (so) she went, and dwelled in the house of her father.