A fancasm is where someone loves a certain person or icon. e.g. L from deathnote or Professor Hershel Layton from professor Layton. They will spend a certain period of time obsessing about this person. e.g. last night I made up my own Professor Layton story in my head (sad right?). Or you just giggle with your fancasm friend for hours. Or make a painting or drawing about them!!
FANCASM
Fan-caz-m
I don't know if it is spelt right sorry!
I hope this helps you either in a big way, or not at all. :)