Green’s Dictionary of Slang

fu n.

[? Sp. fumar, to smoke]

(drugs) marijuana.

[US]C.R. Cooper Here’s to Crime n.p.: Commonly it now being called ‘fu’, ‘mezz’, ‘mu’, ‘moocah’, ‘muggles’, ‘weed’ and ‘reefers’.
[US]D. Maurer ‘Lang. of the Und. Narcotic Addict’ Pt 2 in Lang. Und. (1981) 102/2: fu. 1. Marijuana, or cannabis indica.
[US]Anslinger & Tompkins Traffic In Narcotics 308: fu. Marihuana.
[US]J.E. Schmidt Narcotics Lingo and Lore.
[US]Abel Marihuana Dict.
[US]ONDCP Street Terms 9: Fu — Marijuana.