Green’s Dictionary of Slang

(orig. US) wonderful, the best, outstanding; also used ironically/negatively.

1964		in Current Sl. (1967) I:4 4/2: Marvy-groovy, adj. Bad, unpleasing (esp. sarcastic).
1970		New Yorker 14 Mar. 34: ‘O Sad Arthur, how marvie!’ cried the entranced Lambie. [Ibid.] 17 Oct. 39: Marvy — now we can go on the trip together!
1996		D.F. Wallace ‘David Lynch Keeps His Head’ in Premiere 112/2: This is a genius auteur whose vocabulary in person consists of things like okey-doke and marvy and terrif and gee.
2000		W. Birch No Sleep Till Canvey Island 250: With a bottle of vodka and a senior Service permanently on the go, Lowe would declare that everything on the tour was ‘marvy’.