coarse cotton cloth used for mourning
緦 centers on a specific coarse cotton cloth used in mourning rituals. The meaning is narrow and concrete, tied to funerary tradition.
緦 combines 糸 (thread, cloth) with 思, which likely contributes the sound. The character was created to name a specific type of mourning cloth.
The left side is thread 糸, and the right side is think 思. Picture someone thinking of the deceased while weaving coarse cotton threads into mourning cloth.
For シ, imagine the coarse cloth being used to wipe tears: the soft sound of crying 'shh' cues シ, while the cloth absorbs the sorrow.