Green’s Dictionary of Slang

(drugs) marijuana.

1936		C.R. Cooper Here’s to Crime n.p.: Commonly it is now being called ‘fu’, ‘mezz’, ‘mu’, ‘moocah’, ‘muggles’, ‘weed’ and ‘reefers’.
1940		‘Jargon of Marihuana Addicts’ in AS XV:3 Oct. 336/2: The drug, itself, is known as [...] moocah, mu [...].
1953		Anslinger & Tompkins Traffic In Narcotics 312: [...] mu. Marihuana.
1969		R.R. Lingeman Drugs from A to Z (1970).
2001		ONDCP Street Terms 15: Mu — Marijuana.