Green’s Dictionary of Slang

(UK juv.) absolutely wonderful.

1947		Argus Week-End Mag. 25 Oct. n.p.: Isn’t he simply supersonic! [OED].
1950		A. Buckeridge Jennings Goes To School 9: We had a supersonic time, sir.
1966		B. Jackson Get Your Ass in the Water (1974) 80: They say a man is supposed to reap exactly what he sow, / fast sights, bright lights, and supersonic whores.