This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 莾 is uncertain. It visually combines 艹 (grass) with 奔, but the original semantic and phonetic roles are unclear.
The grass top 艹 over 奔 (run) suggests grass that runs or spreads wildly, but the actual meaning is uncertain.
For ボウ, imagine a bow-shaped blade cutting through overgrown grass: bow -> ボウ, and the grass 莾 is being cleared.