This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
蛼 combines the insect radical 虫 with 車 (vehicle). The exact historical development is uncertain, but it likely originated as a character for a type of insect, possibly a cricket, given the kun reading こおろぎ.
An insect 虫 that makes a sound like a creaking cart 車: imagine a cricket chirping loudly like an old wooden wheel.
For シャ, picture a shy cricket hiding under a cart: shy -> シャ, and the insect chirps softly.