hard wood
槢 centers on hard, dense wood, the kind used for wedges or sturdy wooden tools.
槢 combines 木 (tree/wood) with 習, which likely contributes the sound. The exact historical development is uncertain, but the character is associated with hard wood or a wedge.
The left side is a tree 木, and the right side 習 suggests repeated practice. Imagine a woodworker repeatedly practicing on a piece of hard wood until it becomes a perfect wedge.
For シュウ, picture a shoe made of hard wood: shoe -> シュウ, and the wooden shoe is as tough as a wedge.