This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
蠘 combines 虫 (insect) with 截, which likely contributes sound. The exact historical development is uncertain.
With 虫 on the left, it likely relates to some kind of insect or worm, but the exact meaning is unclear.
For セツ, imagine a set of rare insects: set -> セツ, and the insect radical 虫 hints at the meaning.
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