This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 茝 is uncertain. It visually combines the grass radical 艹 with 臣, but the original component relationship is unclear.
The grass top 艹 over 臣 suggests a plant, but the specific meaning is unclear. Think of it as a mysterious herb.
For サイ, imagine a sigh (サイ) of frustration when you cannot identify this rare plant: sigh -> サイ.