1. [1930s+] anything unpleasant, varying as to context (cf. nasties n. (3)).
2. [1930s+] anyone unpleasant, threatening or scary; often in pl.
3. [mid-19C; 1960s] the vagina.
4. [1960s+] the penis.
5. [1930s+] sexual intercourse; thus do the (big) nasty
[1980s+] to do something unpleasant (to someone), to cause (someone) trouble or problems.
1. [1960s+] (orig. US) to have sexual intercourse.
2. [1960s+] (US black) to perform cunnilingus.
[2000s] (N.Z.) to expose one’s genitalia.
[1930s–40s] to have sexual intercourse.