disappointed
壈 centers on the feeling of disappointment, a sense of letdown or unmet expectations.
壈 combines 土 (earth) with 稟, which likely contributes the sound. The exact historical development is uncertain, but the character is associated with disappointment.
The left side is earth 土, and the right side 稟 looks like a storehouse. Picture a farmer's earth-built storehouse collapsing, leaving only disappointment.
For オン, imagine the disappointed farmer saying 'Oh no!' as the storehouse falls: 'Oh no' -> オン.
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