Green’s Dictionary of Slang

(US drugs) cheap, refined opium residue.

1936		D. Maurer ‘Argot of the Und. Narcotic Addict’ Pt 1 in AS XI:2 125/2: san-lo. The lowest grade of bootleg opium, refined and worked over from yenshee.
1949		Monteleone Criminal Sl. (rev. edn) 199: sam lo [...] sam low Cheap bootleg opium refined from residue; third run opium. [Ibid.] 218: son-lo A cheap grade of opium refined from the residue.
1959		J.E. Schmidt Narcotics Lingo and Lore 168: Son-lo – An inxpensive grade of opium derived from residue.