Green’s Dictionary of Slang

(drugs) uncut, therefore stronger than usual; of heroin or any other drug.

1960		C. Cooper Jr Scene (1996) 12: It’s dyno, baby.
1962		P. Crump Burn, Killer, Burn! 105: That stuff is sure dyno.
1970		G. Cain Blueschild Baby 25: She hands him the two packets and takes the money. ‘How’s the stuff?’ Sun asks her. ‘Dyno’.