This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
箵 combines 竹 (bamboo) with 省, which likely contributes the sound. The exact historical development is uncertain.
The top is bamboo 竹, and the bottom is 省, which can mean 'look back' or 'examine'. Picture a bamboo tool used for inspecting something, though its exact use is unclear.
For セイ, imagine a bamboo inspector saying 'say' as they examine the bamboo: say -> セイ.
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