Is Tuesday named for a one-handed god?
"drug addict," 1923, from junk (n.1) in the narcotics sense + -y (3). Junker in the same sense is recorded from 1922. Junk for "narcotic" is older.
"run-down, seedy, trashy," 1876, from junk (n.1) + -y (2).
: Junkie logic is the ability to justify whatever needs to be done to support an addiction