This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
櫆 combines 木 (tree) with 魁, which likely contributes sound. The exact historical development is uncertain.
The tree radical 木 beside 魁 suggests something wooden, but the meaning is unclear. Think of it as a mysterious wooden object.
For カイ, imagine a wooden chest (kai) made from the tree 木: kai -> カイ, even though the meaning is uncertain.
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