This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
貹 combines 貝 (shell, money) with 生 (life, birth), but its exact historical development and original meaning are uncertain.
The left side is a shell 貝, often associated with money or value, and the right side is life 生. Picture a rare treasure that brings life or vitality, but its exact meaning is uncertain.
For セイ, imagine a safe セイf (safe) where a rare shell treasure is kept: safe -> セイ, and the shell 貝 is the treasure.