When you "mark out" you stand up for something or believe something completely. If you're a massive fan of a band and you refuse to let people critize them, you are marking out.
Believing something completely.
Comes from the wrestling term mark
where a smark
forgets reading the spoilers, guessing the storylines and bitching about backstage and just feels the moment.
1. Dude, you always mark out for crappy emo
2. Yeah, I totally marked out last night when Foley got thrown off the cage.