This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
鼱 is a rare character combining 鼠 (mouse/rat) with 青 (blue/green). The exact historical development is uncertain, but it likely originally referred to a type of rodent, possibly with a bluish or greenish association.
A mouse (鼠) with a blue/green (青) hue: imagine a rare blue-tinted rodent, but its exact meaning is unclear.
For セイ, imagine a blue mouse saying 'say' (セイ) as it squeaks: the mouse's squeak sounds like 'say', tying the blue mouse to セイ.
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