Green’s Dictionary of Slang

1. (US black) of a person, aggressive.

1972		E. Folb Urban Black Argot 138: Foul cold-blooded; ruthless.
1980		E. Folb Runnin’ Down Some Lines 44: You get labeled as too [...] foul and are forced to do something about it.
1993		L. Rodríguez Always Running (1996) 49: Rano [...] was a foul-faced boy in Watts.

2. (US black) of talk, deliberately belligerent [note a foul mouth in the context of obscenity is SE].

1980

E. Folb Runnin’ Down Some Lines 44: Lame talk refers to bad talk, in the sense of provocative or belligerent dialog. Expressions like smart mouth, fat mouth, foul mouth and bad mouth [...] Nasty mouth he’s as foul as he can be!