Green’s Dictionary of Slang

(drugs) a cocaine and morphine mixture.

1950		Goldin et al. DAUL 39/2: C-and-M. A mixture of cocaine and morphine. ‘Them junkeys (drug addicts) hooked on (habituated to) c-and-m would rat (inform) on their mothers for the whizz-bang (potent drug mixture).’.
1957		C. Himes Imabelle 30: Goldy cooked a C and M speedball over the flame.
1965		C. Himes Rage in Harlem (1969) 31: [as 1957].
1971		E.E. Landy Underground Dict. (1972).
2001		ONDCP Street Terms 5: C & M — Cocaine and morphine.