marrow, pith
髓 centers on the soft, vital inner substance of bones and plants: marrow and pith. Both meanings point to the essential core material inside a harder outer structure.
髓 combines 骨 (bone) with 遀, which likely contributes the sound and relates to the idea of something soft or internal. The character is associated with marrow and pith, but the exact historical development of the right component is uncertain.
The left side 骨 means bone, and the right side 遀 looks like a container with something soft inside. Picture cracking open a bone to reveal the soft, vital marrow within.
For ズイ, imagine a zoo where the animals' bones are filled with rich marrow: zoo -> ズイ, and the marrow is the essential core.
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