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1.
An event full of fat people all celebrating their right to be fat and proud.

Something a fat person does without shame
Wow, the summer fat fair was so fat-tastic we really made the world see that underneath all this lard we are human after all.


Did you see the fat-tastic way she just ate 4 chocolate eclairs?
by Fattymuldoon July 13, 2008
 
2.
Fantastically fat person
"That bitch needs to drop a few, but damn she's fattastic!"
by kerklein2 November 15, 2006
 
3.
so amazingly annoying that there can be no other explanation other than the above
"hey i was trying to take a nap and our neighbors were being really FATTASTIC again"- so says Melissa
by kt April 07, 2005