This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
皠 combines 白 (white) with 崔, which likely contributes sound. The exact historical development is uncertain.
The left side 白 suggests whiteness, and the right side 崔 has a mountain-like shape. Imagine a snowy white mountain peak, but remember the meaning is uncertain.
For サイ, picture a sigh of relief when you see a pure white mountain: sigh -> サイ, tied to the white 白 component.