This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
賖 combines 貝 (shell, money) with 余, which may have contributed sound. The exact historical development and original meaning are uncertain.
The left side is a shell (貝), often associated with money or value, and the right side is 余 (surplus). Picture a surplus of shells, but the overall meaning remains unclear.
For シャ, imagine a shy person hiding their surplus shells: shy -> シャ, though the kanji's meaning is uncertain.