And Onan knew that the seed should not be his; and it came to pass, when he went in unto his brother's wife, that he spilled it on the ground, lest that he should give seed to his brother. Onan knew that the seed should not be his - That is, that the child begotten of his brother's widow should be reckoned as the child of his deceased brother, and his name, though the real father of it, should not appear in the genealogical tables.
Related Commentaries of Genesis 38:9
Genesis 38:9
9 And Onan knew that the seed should not be his; and it came to pass, when he went in unto his brother's wife, that he spilled it on the ground, lest that he should give seed to his brother.