Green’s Dictionary of Slang

girl n.2

[the image of cocaine as a ‘feminine’ drug, the injecting of which gives a sexual thrill (although heroin, too, has that effect on some users), as opposed to heroin or boy n.2 (4a), a ‘masculine’ drug, i.e. one that ‘knocks you down’]
[1950s+] (drugs)

1. cocaine.

2. heroin [may be a mis-reading].

In compounds

girlfriend (n.)

see separate entry.