named after Onan
in the Old Testament
, who was commanded by God
to impregnate his brother
so that there would be an heir
for his brother's line. Onan had sex with the woman
, but pulled out and "spilled his seed upon the ground," thereby incurring God's wrath
The word came to mean masturbation because in many branches of Christian theology, any act that involved sperm being wasted was pretty much the same sin.