tie
絏 centers on binding or fastening with a cord or rope: to tie something securely.
絏 combines 糸 (thread) with 曳, which likely contributes the sound and the idea of pulling or dragging, suggesting a cord used for tying or binding.
The left side is thread 糸, and the right side 曳 looks like a hand pulling something. Imagine using thread to pull things together and tie them tightly.
For セツ, picture tying a set of threads into a knot: set -> セツ, and the threads are bound together.
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black rope used to bind criminals; being bound in prison