The term came from a forum where gay males posted stroys about having sex with hiv positive partners, linked below. The term basically means to inject hiv positive(poz) sperm into a hiv negitive(neg) anus(hole).
The word, meaning HIV-positive semen ejaculations, first started on a gay men forum, where the users wanted to find HIV negative people and infect them. While this is serious and they shouldn't be doing this, it is very funny to hear them talk.