crepe
縠 refers to a specific type of silk fabric with a crinkled or crepe-like texture. Its meaning is narrow and technical, centered on this particular textile.
縠 combines 糸 (thread/silk) with a phonetic component that historically indicated the sound. The exact origin of the right-side structure is uncertain, but it likely contributed the reading while 糸 specified the textile meaning.
The left side 糸 shows it's related to thread or fabric. The right side looks like a complex, crinkled pattern, much like the wrinkled surface of crepe silk.
For コク, imagine a bolt of crepe fabric so stiff it feels like a solid block: block -> コク, and the fabric's crinkled texture is as firm as a block.