Poor Hermione!
The phrase had always sounded quite violent and disturbing to me.
Wifey & I had been meaning to find out what this actually meant in England for a long time. I ran it down this week and thought other HP people might be interested in it too.
http://disboards.com/showthread.php?t=1500815&page=3 http://www.phrases.org.uk/meanings/344000.html