Green’s Dictionary of Slang

missy n.

1. (also missee, missey, missie) a young girl, esp. as characterized by servants and sometimes derog.

[UK] in 12th Rep. Hist. MSS. Comm. v Appendix 29: A coach fitt for pretty Missee is not to be found ready made [OED].
[UK]Sporting Times 9 June 1/4: Hold on, missie, not all the soda.
[Ire]Joyce Ulysses 700: ...little chits of missies they have now singing Kathleen Kearney and her like.
[Aus]K. Tennant Battlers 313: Here y’are, Missy?
[US](con. 1920s–30s) J.O. Killens Youngblood (1956) 491: Running smiling sirring mamming seeing-and-not-seeing white missey’s birthday garments.

2. (US gay) an underage boy.

[US]B. Rodgers Queens’ Vernacular 44: any boy under the age of consent [...] missy (camp, kwn LV, mid ’60s: any teenage boy, gay or straight: ‘Hi, missy, wanna come over and have a completely unique experience in depth?’).

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

[US]Simon & Burns Corner (1998) 62: They called it ‘girl’ or ‘Jane’ or ‘Missy’ in feminine contrasts to ‘boy’ or ‘John’ or ‘Mister’ for king heroin.