Green’s Dictionary of Slang

(US drugs) the residue of the narcotic/water solution that remains after it has been strained through a cotton filter.

1938		D. Maurer ‘Lang. of the Und. Narcotic Addict’ Pt 2 in Lang. Und. (1981) 108/1: rinsings. The residue of solution remaining in the cotton after an addict fills his hypodermic.
1949		Monteleone Criminal Sl. (rev. edn).
1959		J.E. Schmidt Narcotics Lingo and Lore.