This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 夝 is uncertain. It visually combines 夕 (evening) and 生 (life), but the original meaning and component relationship are unclear.
With 夕 (evening) and 生 (life), imagine the quiet life of the evening, but since the meaning is uncertain, treat this as a placeholder.
For セイ, picture a serene evening (夕) where life (生) is calm: the word 'say' (セイ) echoes the peaceful scene.