The story takes place in a totally impossible imaginary 'carnival' that mysteriously appears one night. The theme of the carnival is that it caters to the ultimate extremes of sexual fantasy. In this carnival, everything is possible, all fantasies can be realized (it's a fantasy), it is a magical place. The majority of the story takes place of course at that part of the carnival where my fantasies can be realized, the place where willing women can be shot in their bellies with arrows and otherwise killed for the pleasure of the carnival goers.
These women are targets for arrows, they are the 'butt' that the arrow strikes. The word 'butt' is the standard term for an arrow target. As these targets in this case are the bellies of the willing women, the term 'Bellybutt' occurred to me in the writing of the story (the similarity of the word Bellybutt to bellybutton was not lost on me). The term was so apt that it became the title of the story. Subsequently, as the story was so much fun for me to write and ultimately after being posted to the net (a year later), enjoyed by so many, I wrote other stories around the same theme. In these other stories, the women were referred to as 'Bellybutts' as well. A whole series of stories was written that is now referred to as the Bellybutt Chronicles.
As time went on and the stories became popular, the term came to be used by other people as well when referring to women willing to be killed in this manner (at least in fantasy). Art was created (not just by myself) that specifically referred to 'Bellybutts'. The word came to be used not just in correspondence with myself, but also in postings to the Net by other people. It was inevitable that I myself use the term when referring to women in my fantasy. The word may seem strange when first used or heard, but it appears to grow on you.