Mixture of fantastic and fabulous. Makes one sound even more gay than "faaaboulouuuus!" But fun to say and describes something that has no negative aspects to it at all.
Derives from the word fabulous and fantastic. Basically a more fabulous way of saying something is fantastic. (Or vice versa, whatever tickles your fancy)
Ashlee: OMG! You are so fantabulous!
Eric: i kno...
A 2 year-old does a perfect summersault...
Fred: Wow! Thats just how fantabulous my kid is. He does summersaults before he can walk!
Someone pees their pants...
Adam: Now thats fantabulously funny!