Green’s Dictionary of Slang

1. (US Und.) guilty, ‘red-handed’; usu. in phr. caught foul, caught in the act.

1841		N.Y. Daily Trib. 15 July 2/5: Being caught ‘fowl,’ as the parlance is, [he] confessed he stole the pocket book and money, and stated that he was alone in the transaction — which is not true, as two or three always go together, one to steal and the other or others to receive and conceal the property stolen.
1859		Matsell Vocabulum.
1860		in C. Martel A Detective’s Notebook in Partridge DU.
1872		G.P. Burnham Memoirs of the US Secret Service 110: The hitherto lively, busy Canadian [...] owned up that the U.S. Service men ‘had him foul,’ at last.
1927		(con. 1918) J. Stevens Mattock 280: He’ll hook up with her as sure as God made sour apples. She’s got him foul.

2. (orig. US black) aggressively, intemperately.

1993		Source Dec. 51: If you live foul, time is gonna penalize you.
2014		Stormzy ‘Shut Up’ 🎵 Dare one of you man try get loud / All of my mandem move so foul.