This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 甠 is uncertain. It visually combines 日 (sun) and 生 (life), but no clear traditional etymology is available.
With no clear meaning, picture the sun 日 giving life 生 to an unknown idea—like a mysterious ancient symbol whose meaning has been lost.
For セイ, imagine saying 'say' to the sun 日 and life 生, as if trying to name this mysterious kanji: say -> セイ.
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