This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development is uncertain. The modern form combines 竹 (bamboo) with 塞, but the original semantic and phonetic roles are unclear.
The top is bamboo 竹, and the bottom is 塞 (block/close). Picture a bamboo fence used to block a path, but remember its actual meaning is uncertain.
For サイ, imagine a sigh (サイ) of frustration when you encounter a rare kanji with no clear meaning: sigh -> サイ.