thin silk
絅 centers on a specific type of silk: a thin, lightweight silk fabric. The meaning is narrow and concrete, without broad abstract extensions.
絅 combines 糸 (silk thread) with 冋, which likely contributes the sound. The exact historical development of 冋 is uncertain, but the character is used for a kind of thin silk.
The left side 糸 is silk thread. The right side 冋 looks like a thin, open frame. Picture a delicate silk cloth stretched so thin you can see through its open weave.
For ケイ, imagine a thin silk cape: cape -> ケイ, and the cape is made of the lightest silk.