1. [late 19C+] (orig. US drugs) an addict, esp. of opium; usu. in comb. with drug name [contemporary use tends to be ironic].
2. [late 19C–1910s] (US campus) a fool [fig. use of sense 1].
3. [late 19C+] an addict, an obsessive, other than of drugs.
4. on bad = good model.
(a) [1900s] (US campus) a clever student.
(b) [1960s] (US black) a general term of praise for any person or thing.
5. [2000s] someone who smokes marijuana alone (since smoking is usu. a communal experience).
[1930s–50s] (US drugs) an inexperienced or naïve drug user, one who is in the early days of their addiction to narcotics.