Green’s Dictionary of Slang

(drugs) top grade smoking opium.

1892		Morn. Call (S.F.) 4 Dec. 12/4: Li Yurn opium is worth from $16 to $18 per five-tael tin.
1898		L.J. Beck N.Y.’s Chinatown 145: The best No.1 comes from Hong Kong, and it is sold in cans. [...] This is known as Fook Yuen or ‘Fountain of Happiness,’ and Li Yuen, or ‘Fountain of Beauty’.
1916		T. Burke Limehouse Nights 21: It was a well-kept place where one might [...] take a shot or so of li-un.
1936		D. Maurer ‘Argot of the Und. Narcotic Addict’ Pt 1 in AS XI:2 123/2: li-yuen. A high-quality smoking opium.
1949		Monteleone Criminal Sl. (rev. edn).
1959		J.E. Schmidt Narcotics Lingo and Lore.