This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
箲 combines 竹 (bamboo) with 洗 (wash). The exact historical development is uncertain, but it likely originally referred to a bamboo tool used for washing or scrubbing.
Bamboo 竹 over washing 洗: imagine a bamboo scrub brush used for cleaning.
For セン, picture a bamboo scrub brush sending (セン) dirt away as you wash: send -> セン.