Word made up by the character "The Janitor" from the NBC hospital based sit-com Scrubs. The word is said by Bob Kelso in season 5 episode 24 when he is blackmailed into announcing on the hospital PA system that he is afraid of the Janitor.
Bob Kelso: "...so, please join me in basking in the glow of the Janitors awesome... *pauses* ...this isn't a word?!"
A combination of "Fat" and "Attitude"; the loud, boisterous, obnoxious arrogance often displayed by large and/or obese women. This is very common; many large women often try to overshadow their physical enormity with a similarly large and over-exaggerated large-and-in-charge attitude.
"Dude, that blubbery fat chick needs to shut up already. She's got a major case of fattitude."