Usually a group of 10 year old to teenage kids who wear mallcore t-shirts and fake that their lives are awful. They often hang out in groups and are among the "popular" kids, but think that they are alienated and are utterly friendless. While mallcore kids are generally annoying and are shallow, uninteresting people, the mallcore girls are usually pretty hot.
"Mallcore kid," in metal circles, is often used as an insult against someone who doesn't know much about metal, or likes a metal band that is considered to be mallcore. While most metalheads have at least one "mallcore" band that they listen to or enjoy as a guilty pleasure, these cannot be considered mallcore kids because they are, all in all, metalheads.
Mallcore kids can be spotted by their often emo
hairstyle, a "Limp Bizkit," "Korn," or post-Black Album "Metallica" T-shirt. They also tend to say "Slipknot is the heaviest f---ing band ever." (Let me say, right now, that Slipknot is not a mallcore band.) While you might see a kid in one of these T-shirts, ensure to ask them about a real metal band before judging them as a mallcore kid.
If they have emo
or dyed black hair, don't bother.
Most mallcore kids tend to be shallow in their music tastes, so they definitely aren't someone to get involved with if you are a music junky.