This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
紪 combines 糸 (thread) with 此, but the exact historical development and original meaning are uncertain.
The left side is thread 糸, and the right side is 此. Without a clear meaning, picture a rare, uncertain pattern woven into silk.
For セイ, imagine a weaver saying 'say' (セイ) as they examine the mysterious thread pattern.