Green’s Dictionary of Slang

some form of extreme, usu. positive; incredibly wonderful .

1953		Chicago Trib. 23 June 15: [advert] ‘Fantabulous’ is the word for a Delta-C&S vacation.
1959		N.Z. Listener 9 Jan. 4/2 : Air travel – it’s fantabulous [OED].
1961		T. Taylor Baron’s Court All Change (2011) 107: ‘I have to pay a fantabulous rent’.
1963		G. Lambert Inside Daisy Clover (1966) 81: ‘How does it feel to have the world at your feet?’ ‘Fantabulous!’ I said.
1978		A. Maupin Tales of the City (1984) 9: ‘Fantabulous!’ squealed Connie.
1981		(con. 1940s) O’Day & Eells High Times Hard Times 119: ‘One hundred and twenty-five dollars a week.’ ‘Fan-TAB-ulous!’.
1992		K. Lette Llama Parlour 42: Personally it [i.e. Ecstasy] gives me the most fan-tab-u-lous orgasms.
2004		T. & B. Donaldson Coming Of Age 83: So, we did that and looked absolutely fantabulous if I do say so myself.