Green’s Dictionary of Slang

(US black) bootleg liquor, moonshine.

1940		E.F. Frazier Negro Youth 16: [B]ootleg liquor known as ‘mammy,’ ‘splo,’ and ‘derail’ is sold in the dens of vice to which men go for all types of sexual pleasures.
2006		(con. 1940s) B. Martin Both Sides of the Fence 120: Basically [...] when you drank it [i.e. splo] something exploded inside you, usually your head, hence the abbreviated ‘splo’.