This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
荿 is a rare character combining the grass radical 艹 with 成. The exact historical development and original meaning are uncertain.
The top 艹 suggests a plant, and the bottom 成 can mean 'become' or 'achieve'. Imagine a plant that grows into something, but its exact meaning is unclear.
For セイ, picture a plant (艹) that becomes (成) a sage (セイ) — a wise herb. The sage sound cues セイ.
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