Green’s Dictionary of Slang

(orig. US black) wonderful, admirable, excellent.

1948		Down Beat 28 July 4: insane: only the musical literati are addicted to (and permitted to use) this word [...] Applied only when the subject is ‘too gone’ for ‘crazy’ description.
1955		Babs Gonzales ‘The Be-Bop Santa Claus’ 🎵 So, cast thine eyes in my righteous bag and see / What insane object I shall lay on thee!
1958		G. Lea Somewhere There’s Music 165: The City’s insane!
1964		R.S. Gold Jazz Lex. xxii: Currently, in response to an inspired musical performance, a jazz devotee draws from a large storehouse of terms: crazy, nutty, insane, swinging, groovy, cooking, wailing, burning, smoking, boss, something else, out of sight, etc.
1993		M.B. ‘Chopper’ Read How to Shoot Friends 117: I thought, ‘What a totally insane-looking woman.’.
2020		C. von Ziegesar Cobble Hill 229: ‘You're a goddess with a heart-shaped mouth and a heart-shaped face. Even your nose is sort of heart-shaped. So is your butt. And your cleavage. You're insane!’.