In the late 1980's the definition was taken to greater heights by a subculture of NYC and SF club kids. It was used to describe how one felt while on the drug ecstacy, and especially while "candyflipping" - a combination of LSD and pure pharmaceutical ecstacy. To describe a feeling of a state of eternal bliss!
Last night was amazing. The music, the people, and the overall positive energy made me feel simply fantabulous!
(stating one felt fantabulous was code for saying they were on "e")
Fantabulous is an adjective meaning something is of the highest quality. It is a combination of fantastic and fabulous, which was commonly used in London in the 1960s and 70s to describe various aspects of pop culture and fashions. It was especially popular with Chelsea girls like Joanna Lumley. It is still in use in England today, but not as much as it used to be.
That new coat of yours looks absolutely fantabulous!