horse's buttocks, horse's tail, (ghost kanji)
駲 is a rare ghost kanji with no clear standalone meaning. Its listed glosses point to parts of a horse, but treat its meaning as uncertain.
The exact historical development of 駲 is uncertain. It is a rare character combining 馬 (horse) with 州, but the original purpose and component relationship are unclear.
The left side is a horse 馬, and the right side looks like a flowing tail or rump 州. Picture a horse with a distinctive tail or hindquarters.
For シュウ, imagine a horse's tail swishing with a 'shoo' sound: shoo -> シュウ, as the horse flicks its tail.
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