This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
恀 combines 忄 (heart/mind) with 多, but the exact historical development and original meaning are uncertain.
With 忄 (heart) beside 多 (many), imagine a heart full of many feelings, but since the meaning is unclear, treat it as a heart overwhelmed by many things.
For シ, picture a heart (忄) full of many (多) emotions, and think of the sound 'she' as the heart sighs: she -> シ.