black clothing, priest
緇 centers on the color black as it appears in dyed fabric, specifically the black robes worn by Buddhist priests. The priest meaning is a natural extension of that black clothing.
緇 combines 糸 (thread) with 甾, a component that likely contributes the sound and relates to dark or black dyeing. The character originally referred to black-dyed silk or cloth.
The left side is thread 糸, and the right side 甾 looks like a container for dye. Imagine thread being dipped into a vat of black dye, producing black clothing worn by priests.
For シ, picture a priest in black robes quietly saying 'shh' to keep the temple silent: shh -> シ, and the black-clad priest embodies the kanji.