the bank of a steam
滹 refers to the bank or shore of a stream or river. It is a rare character with a narrow, concrete meaning.
滹 combines the water radical 氵 with the phonetic component 虖. The exact historical development is uncertain, but it likely originated as a phono-semantic compound for a specific river or shore feature.
The left side is water 氵, and the right side 虖 looks like a tiger 虍 with a mark underneath. Imagine a tiger standing at the water's edge, marking the bank of a stream.
For コ, picture a small child (子, こ) playing safely on the bank of a stream: 子 -> コ, and the water 氵 flows nearby.
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