Green’s Dictionary of Slang

excellent, good beyond expectation.

1938		M. Allingham Fashion in Shrouds 175: Oh, Val, isn’t it fantastic? [...] It’s amazing, isn’t it?
1967–8		Baker et al. CUSS 115: Fantastic Extraordinary, unusual, hard to believe.
1984		A. Sayle Train to Hell 44: Hey! [...] it’s fantaaastic, rilly, rilly, fantaaastic.
1999		Guardian G2 4 Oct. 6: It’s fantastic they’ve given John Barnes the job. Fan-bloody-tastic.