This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
樨 combines 木 (tree) with 犀, which likely contributes the sound. The exact historical development is uncertain.
The tree radical 木 beside 犀 suggests a type of tree or wood, but the specific meaning is unclear.
For セイ, imagine a rare tree named 'Sei' that grows only in a hidden grove: Sei -> セイ.