Genesis 38:9

9 Aber da Onan wußte, daß der Same nicht sein eigen sein sollte, wenn er einging zu seines Bruders Weib, ließ er's auf die Erde fallen und verderbte es, auf daß er seinem Bruder nicht Samen gäbe.

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 Aber Ger war böse vor dem HERRN; darum tötete ihn der HERR. {~} {~}

8 Da sprach Juda zu Onan: Gehe zu deines Bruders Weib und nimm sie zur Ehe, daß du deinem Bruder Samen erweckest.

9 Aber da Onan wußte, daß der Same nicht sein eigen sein sollte, wenn er einging zu seines Bruders Weib, ließ er's auf die Erde fallen und verderbte es, auf daß er seinem Bruder nicht Samen gäbe.

10 Da gefiel dem HERRN übel, was er tat, und er tötete ihn auch.

11 Da sprach Juda zu seiner Schwiegertochter Thamar: Bleibe eine Witwe in deines Vaters Hause, bis mein Sohn Sela groß wird. Denn er gedachte, vielleicht möchte er auch sterben wie seine Brüder. Also ging Thamar hin und blieb in ihres Vaters Hause.