Green’s Dictionary of Slang

gee yen n.

also yi-yen
[Chinese yí-yín]

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

[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].
[US]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.
[US]Monteleone Criminal Sl. (rev. edn).
[US]J.E. Schmidt Narcotics Lingo and Lore.