dense growth, thicket
蓁 centers on thick, tangled plant growth: a dense thicket or lush, overgrown vegetation.
蓁 combines the grass radical 艹 with 秦, which likely contributes the sound. The character originally referred to thick, overgrown vegetation.
The grass radical 艹 on top of 秦 suggests a place where plants have grown thick and tangled, like a dense thicket.
For シン, imagine a dense thicket so thick you can barely see the shin (shin) of a person walking through it: shin -> シン.
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