Green’s Dictionary of Slang

slit n.

1. with ref. to a woman.

(a) [early 17C+] the vagina.

(b) [1940s+] a derog. term for a woman.

2. [1940s+] (also sliteye) a derog. term for an Asian or East Asian person; thus slit-eyed adj. [the supposed shape of their eye].