bamboo splints or slats
篾 refers to thin strips or slats of bamboo, used as a material for weaving or binding.
篾 combines 竹 (bamboo) with a phonetic element that likely contributed the sound. The exact historical development of the lower component is uncertain.
The top 竹 is bamboo, and the rest looks like thin strips being woven. Picture bamboo split into fine slats.
For ベツ, imagine a bamboo weaver saying 'bets' as they place each slat: bets -> ベツ.
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