Remember all those really campy films from the 1950s such as The Crawling Hand?  Idle Hands is basically the same type of movie, only much sillier, much campier, and in some respects embarrassingly bad.  The filmmakers really don't even bother to explain the whole silly concept; a corporeal, murderous hand on the rampage normally doesn't need a lot of background info, but in this case Anton (Devon Sawa) simply has his own hand somehow possessed without the rogue hand causing all the mischief even finding him.  Only after Anton cuts his hand off, do we return to the more traditional animated, murderous hand on the rampage theme, and that works much better than Anton's constant battle to control what his right hand is doing as long as it is attached to his arm.  As far as the rest of the explanation goes, we have a Druidic priestess (Vivica A. Fox) trying to find and destroy the hand before midnight Druid time, at which time the hand will take a soul into the netherworlds.  In my opinion, this story doesn't even begin to work.