Probably referring to Metallica, an insult which obviously is used by the hollow minded and quick to judge. For one, anyone who uses the word prep is obviously too worried about themselves looking a certain way and has to judge how everyone else looks. Secondly, the song Enter Sandman has nothing to do with emotional problems, then again some are too simple minded to look into anything before the judge it. And lastly, the person who made this probably thinks Slipknot, Mudvayne, and Korn are metal bands, and goes off to attack Metallica, because they can't fight for what they say in real life, they have to make themselves feel better on here.
Whoever created this definition is truly a moron, and should form their own band called Shitallica.