sound of crashing stones, bang!
砰 centers on a loud, sharp impact sound, specifically the noise of stones or heavy objects crashing together. The given meanings both point to this sudden percussive noise.
砰 combines 石 (stone) with 平, which likely contributes the sound. The character was created to represent the sound of stones striking each other.
The left side is a stone 石, and the right side 平 suggests a flat surface. Imagine two flat stones smashing together with a loud bang.
For ホウ, picture a stone being hurled through the air with a loud 'hoe!' sound as it crashes: hoe -> ホウ, and the bang echoes.
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