hemp palm
栟 refers to a specific plant: the hemp palm, a type of palm tree whose fibers are used like hemp.
栟 combines 木 (tree/wood) with 并, which likely contributes a phonetic element. The character specifically denotes the hemp palm, a woody plant.
A tree (木) with fibers that can be used like hemp (并 suggests combining or putting together, as hemp fibers are bundled). Picture a palm tree whose leaves are woven together into strong hemp-like ropes.
For ヘイ, imagine a palm tree swaying in the wind, making a 'hey' sound as its fronds rustle: hey -> ヘイ.
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