This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 秂 is uncertain. It visually combines 禾 (grain) and 人 (person), but this may be a late or variant form without a clear ancient origin.
A person 人 under a grain plant 禾 could suggest a farmer or harvesting, but the kanji has no confirmed meaning, so this is only a visual mnemonic.
For ジン, imagine a person 人 working under the grain 禾, and the grain is so heavy it makes a 'jin' sound when it falls: jin -> ジン.
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