This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
裞 combines 衤 (clothing) with 兑, which likely contributes a phonetic element. The exact historical development is uncertain.
The left side 衤 suggests clothing, and the right side 兑 looks like a person with a large head and legs. Imagine a ceremonial robe used in an ancient ritual, but the exact meaning is unclear.
For セイ, picture a ceremonial robe being laid out for a sacred (セイ) event: sacred -> セイ, tying the clothing radical to the reading.