1) used to insult someone who may be fat
2) used to insult someone because you can't seem to think of a better insult than "Fatty McFat Fat"
Hey look, There's Fatty McFat Fat! Freaking Fatty!
1) a mix of the words beast, in this case really good at something, and phat. Someone is very proficient in something.
Hey man, my wide receiver is so good, that he's obeast!