speed, horses running
駸 centers on swift forward motion, especially of horses: speed, galloping, or running quickly.
駸 combines 馬 (horse) with a phonetic component that historically carried the sound シン and the idea of advancing or progressing. The exact origin of the right side is uncertain, but the character is consistently associated with horses moving swiftly.
A horse 馬 gallops so fast that the right side looks like a blur of legs and mane: 駸 is all about speed.
For シン, imagine a horse running so fast it creates a sharp shin sound as its hooves slice the air: shin -> シン.
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