From where does the word "onanism", another term for masturbation, come?

Explanation

The Book of Genesis contains the passage 38:7-11, which is meant to scare people off masturbation. It is the story of Judas' son, Er, who died childless, and his brother Onan, who had to impregnate Er's widow. During the sexual act, however, Onan withdrew before climax and "spilled his seed". God was not pleased and finally killed him. The term "onanism", another term for masturbation, thus comes from the bible. Today we know that there is no need to cry over "spilled seed", as an average man produces some 50 litres throughout his life.