This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 瘄 is uncertain. It combines the sickness radical 疒 with 昔, but the original semantic or phonetic role of 昔 in this character is unclear.
The sickness enclosure 疒 surrounds 昔 (old times). Imagine an old, forgotten illness from the past, but its exact nature is unclear.
For シャク, imagine a shaman shaking a rattle over the sick person: shak -> シャク, trying to cure the mysterious old illness.