tiny, small, water's edge
芮 centers on smallness and marginality: something tiny, slight, or at the edge. The water's edge sense extends this idea to a narrow, liminal strip of land.
芮 is a phono-semantic compound: 艹 (grass) suggests something small or plant-like, while 内 likely contributes the sound. The exact historical development is uncertain.
Grass 艹 growing inside 内 a tiny crack at the water's edge: a small, marginal plant.
For ゼイ, imagine a tiny zephyr (soft breeze) rustling the grass at the water's edge: zephyr -> ゼイ.
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