jeni, under their law a man had to marry his brother's widow and impregnate her. Any children born would take precedence over his children already born and would be considered as the children of his brother.
Onan did not want this to happen, so he practised coitus interruptus and 'spilled his seed on the ground'. At least, that is the way I have always heard the story interpreted, but I suppose he might have masturbated to make himself unable to perform with the widow. The result was the same - no baby for the widow!
Scottish political mess. Is Devolution working?