Son of a Gun

In a traditional face to face war combat, soldiers shoot randomly at another group of army lining up in front. One bullet happens to hit a guy's ballsack (family jewels/the plums/bollocks, testicles) bringing some shock ridden micrscopic tadpoles with it. This reproductive bullet then hit a female passer-by in the womb and thus a baby is born. Hence the Son of a Gun.