Also spelled "Poseurcore", the word is a rip on all of the bands that think they kick ass, but in real life, they're fucking poseurs. Examples include: Most bands with "-core" at the end of the genre name, all Emo bands, the Warped Tour lineup, and recent Green Day.
Coined in this conversation outside a Hot Topic:
Friend: "Dude, those bands piss me off! Come on, Christcore?! Metalcore?! Give me a break."
Me: "We should just group all those bands together and call it 'Posercore'."