swelling, rising, dispersing
滃 centers on the movement and expansion of water: swelling up, rising, or dispersing outward like a cloud of mist.
滃 combines 氵 (water) with 翁, which likely contributes the sound and may suggest a sense of rising or swelling like an old man's hunched back, but the exact historical development is uncertain.
Water 氵 beside an old man 翁: imagine an old man's swollen joints or a rising mist dispersing around him.
For オウ, picture an old man 翁 swelling up like a water balloon and saying 'Ow!' as it rises: Ow -> オウ.
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