This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
愢 combines the heart radical 忄 with 思 (thought). The exact historical development is uncertain, but it likely relates to mental or emotional states.
The heart radical 忄 beside 思 (think) suggests a heart full of thoughts, but the exact meaning is unclear.
For サイ, imagine a sigh (サイ) escaping from a heart tangled in uncertain thoughts: sigh -> サイ.