Green’s Dictionary of Slang

(US drugs) the residue that collects inside the stem of an opium pipe; it may be collected and then resold.

[1903		J.D. Ball Things Chinese 494: The flame of the lamp passes over it, converting part of it into the so-called smoke, and leaving a solid residuum known as ‘yín-she,’ opium dross, and also ‘yí-yín,’ seconds].
1936		D. Maurer ‘Argot of the Und. Narcotic Addict’ Pt 1 in AS XI:2 121/1: gee-yen. Opium which precipitates in very small quantities in the stem of the pipe. This is retrieved sometimes by the chef who may refine it and sell it again.
1949		Monteleone Criminal Sl. (rev. edn).
1959		J.E. Schmidt Narcotics Lingo and Lore.