This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
筀 combines 竹 (bamboo) with 圭, which likely contributes the sound. The exact historical development is uncertain.
The top part is bamboo 竹, and the bottom is 圭, a shape associated with a jade tablet. Picture a bamboo stalk marked with a jade-like seal, but its meaning remains unclear.
For ケイ, imagine a bamboo stalk shaped like the letter K: K -> ケイ, and the bamboo stands tall.