This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
柡 combines 木 (tree) with 汞, but the historical origin and meaning are uncertain. It may be a variant or a rare character with unclear development.
The left side is a tree 木, and the right side 汞 looks like a container with liquid. Picture a tree with sap or water flowing inside, but its exact meaning is unclear.
For エイ, imagine an 'A' grade tree (エイ sounds like 'A') with a special liquid inside: A -> エイ.
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