Originally, a group of people (especially in the Old West) banded together for purposes of law enforement (and harassing Indians). From <i>posse comitatus</i>, Latin for "power of the county". Posse Comitatus is also the name of various white supremacist organizations that have appeared in the US over the years.
He's got the posse after him because he killed a man.
by LudwigVan December 12, 2003