Green’s Dictionary of Slang

(US black) the buttocks, the posterior.

1937		Z.N. Hurston Their Eyes Were Watching God (1998) 31: Ah just as good as take you out de white folks’ kitchen and set you down on yo’ royal diasticutis.
1966		in DARE II 61/2: (Joking words for parts of the body that you sit on) [...] Diasticurious.