This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 罜 is uncertain. It visually combines 罒 (net) with 主, but the original semantic and phonetic roles are unclear.
The top component 罒 suggests a net, and the bottom 主 might hint at a master or main thing, but without a clear meaning, it is hard to form a reliable mnemonic. Imagine a rare net used by a master fisherman, but treat this as a guess.
For シュ, imagine a shoe caught in a net: shoe -> シュ, and the net 罒 traps the shoe.