a prefix used to describe something fat, or someone who looks like a fat version of someone else
"that girl looks like Lindsay Lohan, only FAT, I'm gonna call her 'Falindsay'"
"hey, that guy looks like Tom Cruise, only FAT, I'm gonna call him 'Fatom'... oh, wait a sec ... that IS Tom Cruise!"
"that old lady looks like my grandma, only FAT, I'm gonna call her 'Fagrandma'"
"That is one big fat chicken ... it's a fachicken."