This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
瘛 combines 疒 (sickness) with 恝, but the exact historical development and original meaning are uncertain.
The sickness enclosure 疒 surrounds 恝, a shape that looks like a person with a troubled heart. Picture a mysterious illness causing inner turmoil.
For セイ, imagine a person saying 'say' as they try to describe the strange sickness: say -> セイ.