chip (of wood)
榾 centers on a small piece of wood, specifically a chip or splinter broken off from a larger piece.
榾 combines 木 (tree, wood) with 骨 (bone), likely suggesting a hard, bone-like fragment of wood. The exact historical development is uncertain.
The left side is a tree 木, and the right side is a bone 骨. Picture a bone-dry branch snapping off, leaving a hard wooden chip like a bone fragment.
For コツ, imagine a wood chip flying off with a sharp 'kots!' sound as it hits the floor: kots -> コツ.