This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
蔤 combines the grass radical 艹 with 密, but the exact historical development and original meaning are uncertain.
The top grass radical 艹 over 密 (dense, secret) suggests something hidden or dense in vegetation, but no clear meaning is known.
For ビツ, imagine a hidden dense thicket of grass where you might find bits of secret plants: bits -> ビツ.
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