I'm sure it came from America, but I'm not exactly sure about the "watermelon thing." I have seen on the History Channel that slave owners in the southern US used to give their slaves a chicken on holidays, like Christmas, as a special gift for a meal. The slaves would kill it and fry it since that was the easiest, cheapest and most readily available method for them to cook with since only the wealthy had ovens back in the day. So, I figure that's where the "fried chicken thing" came from. I'm pretty sure that watermelons grow, or at least, used to be grown in the south because of it's long growing season, so maybe the slaves had access to them, too. Maybe they were allowed to grow them in their own gardens. I think The Seal is right. I didn't think about the high water content. The slaves probably needed more water than anyone else because they worked so long and hard in the hot sun, in the hot summers in the south and would probably become dehydrated more easily because of it. I'm surprised that so many have never heard this stereotype before, though I can understand why you in England wouldn't have.