surname
佘 is a rare kanji used primarily as a surname. It has no clear standalone meaning beyond that function.
The historical origin of 佘 is uncertain. It appears to combine a person-related top with 示 (altar, show) at the bottom, but the exact development is unclear.
A person (top) standing before an altar 示, perhaps representing a family name tied to ritual or ancestral worship.
For シャ, imagine a person introducing their surname with a sharp 'shah!' sound at the altar: shah -> シャ.