This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development is uncertain. The character combines 角 (horn) and 氏, but the original semantic and phonetic roles are unclear.
The left side is a horn 角, and the right side is 氏. Without a clear meaning, picture a mysterious horn-shaped sake cup used in an ancient clan ceremony.
For シ, imagine a single drop of sake falling from the horn cup: the soft 'shh' sound cues シ.