Comes from the term Falcon Punch and munt. When someone is about to munt (vomit) they scream Falcon Munt! as loud as they can, and attempt to munt in the most impressive way they can (ie, uberprojectile).
Dude, watch this - FALCON MUNT!
Omg dude! That was awesome!
See falcon punch, munt, vomit, sick, chunder, spew