This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
糦 combines 米 (rice) with 喜 (joy), but the exact historical development is uncertain. It may have originally referred to a type of food or offering, but no clear meaning is preserved in modern Japanese.
The left side is rice 米, and the right side is joy 喜. Imagine a joyful feast with rice, but remember that this kanji's actual meaning is unclear.
For シ, picture a joyful feast where everyone says 'she' (シ) brought the rice. The sound 'she' cues シ, and the rice 米 and joy 喜 tie it to the kanji's shape.