Green’s Dictionary of Slang

nasty n.

[in sexual contexts an acknowledgment, if not an agreement, with the Western ambivalence as regards sexuality; note Williams (1994) for 17C use of nasty as ‘bedfellow’]

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

In phrases

do (someone) a nasty (v.)

[1980s+] to do something unpleasant (to someone), to cause (someone) trouble or problems.

do the (big) nasty (v.)

1. [1960s+] (orig. US) to have sexual intercourse.

2. [1960s+] (US black) to perform cunnilingus.

flash one’s nasty (v.)

[2000s] (N.Z.) to expose one’s genitalia.

have a bit of (old) nasty (v.)

[1930s–40s] to have sexual intercourse.