This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
笇 combines 竹 (bamboo) with 卞, but the exact historical development and original meaning are uncertain.
With bamboo 竹 above and 卞 below, 笇 looks like a bamboo tool or tally, but its meaning is unclear.
For サン, imagine counting with bamboo sticks under the sun: sun -> サン, and the bamboo tool helps count.