can't move, to falter
趦 centers on being unable to move forward: faltering, hesitating, or being stuck in place.
趦 combines 走 (run/move) with 咨, which likely contributes the sound and the idea of hesitating or being unable to proceed. The exact historical development is uncertain.
The 走 (run) enclosure surrounds 咨, which looks like a person stuck in indecision. Imagine trying to run but your legs falter and you can't move.
For シ, picture a person frozen mid-step, whispering 'shh' as they falter: shh -> シ, the sound of hesitation.