Green’s Dictionary of Slang

(drugs) a narcotics user who injects into the skin rather into a vein; the assumption is that such a user is less wholly habituated.

1953		Kramer & Karr Teen-Age Gangs 35: A very expert skin-popper, Hoppy is.
1969		C. Himes Blind Man with a Pistol (1971) 89: The nose pinching smell of marijuana, the grunting and groaning of skin poppers and homosexuals.
1977		D. Powis Signs of Crime 201: [...] the words ‘skin popper’ may be used as a term of patronising contempt by a confirmed addict to one just starting.
2001		(con. 1964–8) J. Ellroy Cold Six Thousand 376: White horse hits. The word expands. Clients accrue: Sniffers/tasters/junkies/skin-popper geeks.