peep at, watch, spy on
覷 centers on observing someone or something secretly or intently: peeping at, watching, or spying on.
覷 combines 虚 (void, emptiness) with 見 (see), suggesting looking into emptiness or peeping secretly. The exact historical development is uncertain, but the phono-semantic structure is plausible.
The left side 虚 suggests a hollow or hidden space, and the right side 見 is an eye. Imagine peeping through a crack into an empty room to spy on someone.
For ソ, picture a spy peeping so quietly that the only sound is a soft 'so...' of breath: so -> ソ.