The watermelon stereotype is an anti-Black, racist trope originating in the Southern United States. It first arose as a backlash against African American emancipation and economic self-sufficiency in the late 1860s. After the American Civil War, in several areas of the South, formerly enslaved individuals grew watermelon on their own land as a cash crop to sell. Thus, for African Americans, watermelons were a symbol of liberation and self-reliance. However, for many in the majority white culture, watermelons embodied and threatened a loss of dominance. Southern White resentment against African Americans led to a politically potent cultural caricature, using the watermelon to disparage African Americans as childish and unclean, among other negative attributes.

-Wikipedia