rough, indelicate silk
絁 centers on a type of silk that is coarse or unrefined, describing rough, indelicate silk fabric.
絁 combines 糸 (silk thread) with 㐌, which likely contributes the sound. The exact historical development of 㐌 is uncertain, but the character as a whole denotes a coarse silk.
The left side is 糸 (silk thread), and the right side 㐌 looks like a rough, uneven shape. Imagine silk threads that are tangled and coarse, producing rough, indelicate silk.
For シ, picture a sheet of rough silk: sheet -> シ, and the coarse texture makes it feel like an old, indelicate sheet.