Green’s Dictionary of Slang

nasty adv.

1. (Aus.) fast.

[Aus]Bulletin (Sydney) 12 Jan. 14/3: They were going very nasty, / With their heads laid to the creek, / My mouth so full of dust and oaths / I couldn’t make it speak.

2. (US teen) extremely (whether pleasant or unpleasant), e.g. nasty cool, very cool indeed.

[UK]R.S. Surtees Hillingdon Hall I 49: I’m not nasty particular, and never was about making money.
[US]V.E. Smith Jones Men 57: They really caught him nasty.

3. (US) of language, obscenely.

[US]D. Jenkins Semi-Tough 206: [S]he mostly liked to smoke dope and talk nasty and fuck athletes.
[US]C. Cook Robbers (2001) 248: Wild woman, liked to drink and talk nasty.