Green’s Dictionary of Slang

a general excl. of approbation.

1965		J.P. Carstairs Concrete Kimono 17: She murmured ‘Fab’ every now and then and was utterly entranced.
1999		Observer Screen 17 Oct. 20: He spends the programme saying ‘Mega fab!’.
2001		Indep. Rev. 20 Feb. 3: ‘Spiffing’ or ‘fab’ or ‘far out’.