This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
榯 combines 木 (tree) with 時, which likely contributes the sound. The exact historical development is uncertain, and the character is rare.
The tree radical 木 beside 時 suggests a tree related to time, but the exact meaning is unclear. Imagine a tree that marks the passing of seasons.
For シ, picture a tree that grows in a single season: the tree (木) and time (時) combine, and the reading シ is like the first sound of 'season'.