Green’s Dictionary of Slang

(Aus./US campus) exceptionally wonderful.

1946		Poughkeepsie Jrnl (NY) 20 Oct. 3A/2: Vocabulary for Squares. Pos Atomic . . . Terrific.
1947		Argus (Melbourne) 15 Nov. 7/1: Hoping that things are atomic and neat as a pleat around your way.
1951		E. Waters His Eye Is on the Sparrow 22: These kids [...] accepted me as an equal, not only because I was hep and so big for my age, but because they were awed by my atomic delivery of the King’s profanity.
1998		Eble Campus Sl. Apr.