archer's arm protector
靹 centers on a protective leather guard worn on the arm by an archer to shield against the bowstring.
靹 combines 革 (leather) with 内 (inside), likely indicating a leather item worn on the inner arm. The exact historical development is uncertain.
Leather 革 worn on the inside 内 of the arm: an archer's arm protector.
For ケツ, imagine an archer named Kets who always wears his leather arm guard: Kets -> ケツ.
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