drift, flow, overflowing, vast
瀁 centers on water in expansive, unconfined motion: drifting, flowing, overflowing, or spreading out vastly. The meanings share the idea of water moving freely without fixed boundaries.
瀁 combines the water radical 氵 with 養, which likely contributes the sound ヨウ. The exact historical development is uncertain, but the structure suggests a phono-semantic compound where 氵 indicates water and 養 gives the reading.
The water radical 氵 beside 養 (which can suggest nourishing or fostering) creates an image of water being nurtured until it overflows and drifts freely, becoming vast.
For ヨウ, imagine a vast ocean where you drift: you -> ヨウ, floating on the overflowing water of 瀁.
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wide and deep (of a body of water)