In Qin and Han times the northern peoples were called Di.
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Read dí, the original sense is a long-tailed pheasant and its feathers, waved in ancient ritual dances. As a name 翟 was an old word for northern peoples, the same as 狄.
羽 is the feather radical on top. The character first meant a long-tailed pheasant, so the bird's plumes sit above; the surname use borrows that older graph.