Referred to a African-American person, calling them a "Tree Dangler", is like calling them a monkey who dangles from trees to pick bananas, or any fruit or food of some sort that grows on trees.
Whiz Khalifa: "Uhhh, you know what it is! black and yellow, black and yellow,black and yellow!"
White Guy: "This nigga right here looks like a Tree Dangler, look at them long monkey arms, what he do? Outstretch em when he was picking bananas?