Junkie = a heroin addict.
Specifically, someone who injects and is addicted to, or "strung out" on heroin.
Not to be confused with "Junky" which can refer to something of little use, value or sometimes a person who is "addicted" to something.
The word Junkie became popular from the book "Junkie" by
William S. Burroughs the worlds most famous and revered Junkie.
Junkie is often used for someone who can't stop doing something, (drugs,
video games, etc) but the true meaning refers only and specifically to a heroin addict.