net for catching rabbits
罝 refers specifically to a net used for catching rabbits. The meaning is narrow and concrete, centered on a hunting tool.
罝 combines 罒 (net) with 且, which likely contributes a phonetic element. The character is a specific term for a rabbit net.
The top 罒 is a net, and the bottom 且 looks like a shelf or a trap. Imagine setting a net over a rabbit burrow to catch rabbits.
For シャ, picture a rabbit caught in the net letting out a soft 'shah' sound as it is trapped: shah -> シャ.