archer's arm protector, (kokuji)
鞆 is a specialized kokuji for a traditional archer's arm protector, typically made of leather. Its meaning is narrow and concrete, centered on that specific piece of archery equipment.
鞆 is a kokuji (Japan-made kanji) combining 革 (leather) with 丙, which likely contributes a phonetic element. It was created to represent the leather arm protector used in Japanese archery.
The left side 革 means leather, and the right side 丙 looks like a person with a protective plate. Imagine a leather guard strapped to an archer's arm: 鞆 is that arm protector.
For とも, picture a friend (友, とも) helping you put on the leather arm guard before archery practice: 友 -> とも, and the guard protects your arm.
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archer's left-wrist protector