NET
9 But Onan knew that the child would not be considered his. So whenever he slept with his brother’s wife, he wasted his emission on the ground so as not to give his brother a descendant.
Darby
9 But when Onan knew that the seed should not be his own, it came to pass when he went in to his brother's wife, that he spilled [it] on the ground, in order to give no seed to his brother.
KJV
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.