This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 莬 is uncertain. It visually combines the grass radical 艹 with 免, but the original semantic and phonetic roles are unclear.
The top 艹 suggests grass or plants, and the bottom 免 looks like a person escaping. Imagine a plant that grows so fast it seems to escape the ground, but its exact meaning remains a mystery.
For ブン, picture a bunch of mysterious grass: bunch -> ブン, and the grass top 艹 hides the unknown meaning.
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