This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
筪 combines 竹 (bamboo) with 匣 (box), suggesting a bamboo box or container. The exact historical development is uncertain.
Bamboo 竹 on top of a box 匣: imagine a woven bamboo box used for storage.
For コウ, picture a bamboo box with a lid that makes a hollow 'koh' sound when closed: koh -> コウ.