To what does the word drop in the "one drop rule" refer?

Name: To what does the word drop in the "one drop rule" refer?
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The "one drop rule" refers to the belief that anyone with even a single drop of "black blood" (meaning any African ancestry) is considered to be black. In this context, "drop" refers to a small amount of blood, specifically the idea that even a tiny amount of African ancestry makes someone black, regardless of their appearance or other racial/ethnic heritage.

To What Does the Word Drop in the "One Drop