Getting really excited for something beyond logic or reason.
This is from an extension of the wrestling fan phrase and has grown into the modern lexicon. It usually comes from something you enjoyed in your childhood that you still enjoy into adulthood without logical reason.
"Dude, I totally am marking out for the RUSH concert"
"Man, when I saw Ozzy Osbourne... I started marking out"
"Theo Fleury is trying to make a come-back? I'm totally marking out dude!"