Brass Ankles is a somewhat derogatory term used in the American South for someone of mixed blood, specifically someone who looks white, but has some black (or sometimes Native American) ancestry.
The original Brass Ankles lived in South Carolina, in the area of Orangeburg County, Berkeley County and Charleston County. They were said to have both American Indian and black blood, and were identified as such regardless of color or known family tree. Having a family name known to be of the Brass Ankles was enough to 'prove' your ancestry. In the 1800s through to the 1950s this was a important distinction, and different areas had different terms for those of mixed ancestry, including Melungeon, Redbones, Red Legs, Buckheads, Coppershanks, Marlborough Blues, or if you were well-educated and polite, Mestizos. Over time, Brass Ankles became one of the most common of these terms, and is still used today in much of Appalachia. Be warned, it is most emphatically not a nice term.
In the early and mid-1900s, having black blood in your family tree was enough to get you ostracized, and there were segregated schools just for those who looked white, but probably weren't really All-The-Way white. In communities that were identified as Brass Anklers, passing as white was not an option, no matter how white you looked.
Although most educated people do not use the term Brass Ankles any more, it may still be familiar to some from the 1931 play Brass Ankle by Charleston playwright DuBose Heyward, about a white family discovering that they had some black ancestry, and the angst resulting from this discovery.