the open seam of a garment which allows freedom of movement
衩 centers on an opening in clothing that allows ease of movement: the open seam or slit of a garment.
衩 combines 衤 (clothing) with 叉, which likely contributes both sound and the idea of a fork or branching opening, suggesting a slit in a garment.
The clothing radical 衤 beside 叉, which looks like a forked opening, suggests a slit or open seam in a garment that lets you move freely.
For サイ, imagine a sigh of relief as you put on a garment with an open seam that lets you move: sigh -> サイ.
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