Harry Potter fan's pet-form (if you
will) of Lord Voldemort, the villian of the series. Used in refering to the Dark Lord in a casual or capricious way, which we can do because we're not terrified of him like the wizarding world of the books is (no wizard in the books would dare utter the
name "Voldy", because it would be the rough equivalent of calling Hitler "Hitley" or something---basically, it just wouldn't fly). Also probably used because "Voldy" is a lot shorter than "Voldemort".