Genesis 38:9 - Levirate Matrimony & Onanism

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.

The seed should not be his because the first born son would carry on the name of Er so that his dead brother's name would not be forgotten in Israel (Deuteronomy 25:6). Spilled it on the ground is considered a form of contraception from which we get the term "onanism." Although Onan did not masturbate, masturbation ends with the same result - a wasted ejaculation.