type of vetch
芫 refers to a specific type of vetch, a plant in the legume family. Its meaning is narrow and botanical, without broader abstract extensions.
芫 combines the plant radical 艹 with 元, which likely contributes a phonetic element. The exact historical development is uncertain, but it is a typical phono-semantic compound for a plant name.
The grass radical 艹 sits above 元 (origin). Picture the original vetch plant sprouting from the ground: 艹 + 元 = a type of vetch.
For ゲン, imagine a vetch plant growing at the edge of a garden, and a gardener says 'Again?' (ゲン) as it keeps coming back. Again -> ゲン.
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blister beetle