long, numerous, marsh plant whose root is used for medicine
莘 centers on abundance and elongation: something long, numerous, or a prolific marsh plant. The medicinal root sense is a specific application of that plant.
莘 combines 艹 (grass/plant) with 辛, which likely contributes the sound. The character originally referred to a long, numerous marsh plant.
The top 艹 suggests a plant, and the bottom 辛 looks like a tall, upright shape. Picture a marsh full of long, numerous stalks: 莘 is that abundant plant.
For シン, imagine a thin, long marsh plant swaying: thin -> シン, and the plant's slender shape matches the meaning.
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