grave clothes
襚 centers on clothing specifically for burial: garments placed with the deceased or used to dress a body for the grave.
襚 combines the clothing radical 衤 with 遂, which likely provides the sound and may carry a sense of completion or sending off, suiting the idea of final burial garments.
The left side 衤 is clothing, and the right side 遂 suggests something being sent off. Picture a set of clothes being sent along with the deceased into the grave.
For スイ, imagine a burial suit being prepared: suit -> スイ, and the suit is the grave clothes.
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