According to Nanny, who or what is the “mule uh de world “?

From the text:

"Honey, de white man is de ruler of everything as fur as Ah been able tuh find out. Maybe it's some place way off in de ocean where de black man is in power, but we don't know nothin' but what we see…De nigger woman is de mule uh de world so fur as Ah can see."