This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 蠞 is uncertain. It visually combines 節 (top) and 虫 (bottom), but the original semantic and phonetic roles are unclear.
The top part 節 suggests a joint or segment, and the bottom 虫 is an insect. Picture a segmented insect, like a centipede, but remember that the actual meaning is uncertain.
For セツ, imagine a segmented insect sitting on a set of leaves: set -> セツ, and the insect rests on the set.
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