This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
棌 combines 木 (tree) with 采, which may have contributed sound. The exact historical development is uncertain.
The left side is a tree 木, and the right side 采 looks like a hand picking from a tree. This suggests something related to wood or trees, but its exact meaning is unclear.
For サイ, imagine a sighing tree: sigh -> サイ, as the tree 木 stands beside the picking hand 采.