wasting disease
瘵 centers on a severe, progressive illness that causes the body to waste away. The given meaning 'wasting disease' captures this idea of gradual physical decline.
瘵 combines 疒 (sickness) with 祭, which likely contributes the sound and may relate to the idea of a serious, perhaps ritualized, illness. The exact historical development is uncertain.
The sickness enclosure 疒 surrounds 祭, a shape associated with rituals. Imagine a wasting disease so severe that it feels like a slow, sacrificial decline of the body.
For サイ, picture a person sighing (サイ) weakly as the wasting disease takes hold: sigh -> サイ, and the sickness enclosure 疒 wraps around the fading body.