Genesis 38:9

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 [it] upon the ground, so that he should not give seed to his brother's wife.

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 offspring wouldn’t be his; and when he went in to his brother’s wife, he spilled his semen on the ground, lest he should give offspring to his brother.