Green’s Dictionary of Slang

joker n.2

(US drugs)

1. a hypodermic syringe.

[US]H. Leverage ‘Dict. Und.’ in Flynn’s mag.
[US]Anslinger & Tompkins Traffic In Narcotics 311: joker. A hypodermic needle.

2. an injection of morphine.

[US]H. Leverage ‘Dict. Und.’ in Flynn’s mag. cited in Partridge DU (1949).
[US]Anslinger & Tompkins Traffic In Narcotics 311: joker. [...] a dose of morphine taken by injection.