sackcloth worn on breast
縗 refers specifically to a coarse sackcloth garment worn on the breast, traditionally as a mourning attire.
縗 combines 糸 (thread, cloth) with 衰, which likely contributes the sound and the idea of decline or mourning. The character is historically associated with coarse mourning garments.
The thread radical 糸 on the left suggests fabric, and the right side 衰 (decline, weaken) evokes the somber, rough texture of mourning sackcloth. Picture coarse threads woven into a breast cloth worn during times of loss.
For サイ, imagine a sigh escaping as you put on the rough sackcloth: sigh -> サイ, and the coarse fabric brushes against your chest.
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