Green’s Dictionary of Slang

missy n.

1. [late 17C+] (also missee, missey, missie) a young girl, esp. as characterized by servants and sometimes derog.

2. [1960s–70s] (US gay) an underage boy.

3. [1990s+] (US drugs) cocaine [note cocaine is a ‘feminine’ drug, see girl n.2 ].