stony hill, stony mountain
砠 centers on a rocky or stony hill or mountain, emphasizing the rough, stone-covered nature of the terrain.
砠 combines 石 (stone) with 且, which likely contributes the sound. The character expresses a stony hill or mountain through the stone component.
The left side 石 means stone, and the right side 且 looks like a pile of rocks stacked up. Together they form a stony hill or mountain.
For ソ, imagine a stony hill so rough that you can only say 'so?' when someone asks if you want to climb it: so -> ソ.
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