turquoise kingfisher
鷫 refers to a specific bird: the turquoise kingfisher. Its meaning is narrow and ornithological, with no common abstract extensions.
鷫 combines 肅 (likely a phonetic component) with 鳥 (bird), forming a character for a specific bird species. The exact historical development is uncertain.
The left side 肅 looks stern and upright, like a solemn guard. The right side 鳥 is a bird. Picture a turquoise kingfisher standing rigidly on a branch, as if on solemn watch.
For シュク, imagine a turquoise kingfisher perched on a shoe rack: shoe rack -> シュク, and the bird's solemn pose fits the left component 肅.
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