to waste, to destroy, to diminish (hence it came to mean a rat)
秏 centers on wasting or destroying something, causing it to diminish. The extended meaning of 'rat' likely arose from the idea of a creature that wastes or destroys stored grain.
秏 combines 禾 (grain) and 毛 (hair/fur), but the historical origin is uncertain. It may have originally depicted grain being wasted or destroyed, with 毛 possibly contributing a phonetic element, but this is speculative.
Grain 禾 being wasted or destroyed, leaving only fur 毛 behind, like a rat that has eaten all the grain.
For コウ, imagine a rat wasting grain in a storehouse, and the owner shouting 'KOU!' in frustration: KOU -> コウ.
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