fringe
紞 centers on a decorative border of hanging threads or cords: a fringe. The given meaning is narrow, so treat it as a specific ornamental trim rather than a broad concept.
紞 combines 糸 (thread) with 冘, which likely contributes the sound. The character is used for a silk fringe or cord, fitting the thread component.
The left side is thread 糸, and the right side 冘 looks like a dangling ornament. Picture a silk cord with a decorative fringe hanging from it.
For タン, imagine a tan-colored fringe on a silk scarf: tan -> タン, and the thread 糸 makes the fringe.