This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
繟 combines 糸 (thread) with 單, which likely contributes sound. The exact historical development is uncertain.
The left side is thread 糸, and the right side is 單, a shape often associated with simplicity or single items. Picture a single loose thread hanging from a garment, suggesting something slack or relaxed.
For セン, imagine a single loose thread (繟) being sent (セン) through a needle: send -> セン.