Green’s Dictionary of Slang

extremely fantastic, an intensified form of SE.

1988		M. Atwood Cat’s Eye (1989) 280: ‘It sucks,’ they say, or very occasionally, ‘fan-fuckin’-tastic.’.
1996		K. Lette Mad Cows 6: Fan-fucking-tastic, whinged Maddy to a morose flock of tuxedoed penguins advertsing the freezer section.
1997		C. Hiaasen Lucky You 98: ‘Not jes good,’ said his ponytailed friend, ‘fan-fucking-tastic!’.
2000		N. Barlay Crumple Zone 146: Fanfuckintastic acoustics.
2012		B. Willoughby ‘Spelled with a K’ in ThugLit Oct. [ebook] [T]he fan-fucking-tastic fact that this asshole knows my name.
2021		‘Aidan Truhen’ Seven Demons 132: We are the Class Army for Nature [...] it is fanfuckingtastic’.