state in Henan province
鄀 is a rare kanji referring to an ancient state located in what is now Henan province. Its standalone meaning is limited to this historical place name.
鄀 combines 若 (possibly phonetic) with the city/walled-town radical ⻏, typical of ancient place names. The exact historical development is uncertain.
The left side 若 suggests young plants or compliance, and the right side ⻏ is a city wall. Picture a young settlement growing into a walled state in Henan.
For ジャク, imagine a young (若) city (⻏) named Jak, like a small ancient town: Jak -> ジャク.
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