Green’s Dictionary of Slang

(US) a drug addict; also as adj.

1971		L. Cameron First Blood 85: I know a junko when I see one....He’s either on horse or snow [HDAS].
1976		Dr John [song title] Junko Partner.
1994		Rebennack & Rummel Under A Hoodoo Moon 31: I had a lot of junko partners, but Shank was the cat who turned me on to drugs.