Green’s Dictionary of Slang

(US drugs) a pellet of prepared opium.

1882		A. Trumble Heathen Chinee 31: For this money, the Chinaman gives from six to ten ‘fun’ (thirty-two to sixty-four grains) of No. 1 or first class [...] opium.
1920		F. Williams Hop-Heads 112: Two foon of hop retails in the San Francisco joints at four bits; five foon at $1.00.
1936		D. Maurer ‘Argot of the Und. Narcotic Addict’ Pt 1 in AS XI:2 121/1: fun (Usually pronounced foon.) A ration or pill of opium.
1949		Monteleone Criminal Sl. (rev. edn).
1959		J.E. Schmidt Narcotics Lingo and Lore.