tremble, shiver, shudder, quiver
哆 centers on involuntary shaking: trembling, shivering, shuddering, or quivering, often from cold, fear, or strong emotion.
哆 combines 口 (mouth) with 多, which likely contributes the sound and may suggest a wide-open or trembling mouth. The exact historical development is uncertain.
A mouth 口 that is many 多 times opening and closing, like chattering teeth when you tremble or shiver.
For シ, imagine shivering so hard your teeth chatter with a sharp 'shh' sound: shh -> シ, and your mouth trembles.