knot in wood
樶 centers on a hard, irregular growth in wood: a knot. The meaning is specific to this physical feature in timber.
樶 combines 木 (tree, wood) with 最, which likely contributes the sound サイ. The character was created to name a knot in wood, but the exact historical development is uncertain.
The left side is a tree 木, and the right side 最 suggests something extreme or prominent. Picture the most noticeable hard lump on a piece of wood: that is the knot.
For サイ, imagine a sigh of frustration when your saw hits a knot in the wood: sigh -> サイ, and the knot stops the blade.