Right side
厶 supplies an exact phonetic match: sī → sī, no drift.
厶 is the archaic form of
私 itself — a coiled shape representing 'enclosed for oneself'. The classical contrast:
公 (public) is graphically
八 (split open) over
厶 (private) — breaking apart what was held selfishly. The grain radical was added later to disambiguate.
