Green’s Dictionary of Slang

skinpop v.

also pop, skin
[SE skin + pop v.1 (4a)]

1. [1940s+] (drugs) to inject a narcotic beneath the skin rather than directly into a vein; the effect of such an injection is less immediate and somewhat weaker; thus skin-popping n.

2. [1980s] (drugs) the scar left from such an injection.