Belongs to the family of swear words that include parts of the human anatomy, such as 'shithead' and 'bitchface'.
It is both a noun and just a general expletive.
Voldemort: Harry, I killed your parents, and now I shall kill you. ~Struts towards Harry~
Harry: Just piss off you fuckerknuckle.
Emma: They're making 50 Shades of Grey into a film.
Hayley: Oh fuckerknuckle! Why?!!?