This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
柦 combines 木 (tree/wood) with 旦, which likely contributes the sound. The exact historical development is uncertain.
A wooden (木) object beside the dawn (旦) — perhaps a tool or frame used at daybreak, but its exact purpose is lost.
For タン, imagine a tan-colored wooden (木) frame catching the first light of dawn (旦): tan -> タン.
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