This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 冣 is uncertain. It visually combines 冖 (cover) and 取 (take), but the original semantic and phonetic roles are unclear.
The top 冖 is a cover, and the bottom 取 means take. Picture taking something and covering it, but the exact meaning remains unclear.
For サイ, imagine a sigh (サイ) of frustration when you cannot figure out what this rare kanji means: sigh -> サイ.