Proper use of the archaic term is an old reference to unknown ancestry or ancestral traits which may show up in later generations dating back to the days of slavery (and even afterward) when interracial affairs were much more common than people liked to believe.
Former S.C. Senator Strom Thurmond admitted siring a child by a black woman in his later years and provided both her and the child support. The woman was classy enough to keep quiet about what would have been a career ender.
W.C. Fields used the phrase “Ubangi in the fuel supply” in a movie in the 30s. Back then everyone knew what the original phrase meant.