hem or border of a garment, tassel
繸 centers on a decorative or functional edge made of thread: the hem or border of a garment, or a tassel hanging from fabric.
繸 combines the thread radical 糸 with the phonetic component 遂, which likely contributes the sound and may carry a sense of following or trailing, fitting the idea of a tassel or hem that trails along fabric.
The left side 糸 is thread, and the right side 遂 looks like something trailing or following. Picture a thread trailing along the edge of a garment, forming a neat hem or a dangling tassel.
For スイ, imagine sewing a tassel onto a suit: suit -> スイ, and the thread 糸 follows the edge of the fabric.