unbarked lumber
榑 centers on wood in its raw, unprocessed state: lumber that still has its bark, not yet planed or finished.
榑 combines 木 (tree, wood) with 尃, which likely contributes the sound. The character specifically denotes wood with its bark still attached.
The left side 木 is a tree, and the right side 尃 looks like a hand spreading something over a field. Picture a freshly cut log with bark still spread over it, like a rough, unbarked piece of lumber.
For フ, imagine a huge, unbarked log that makes a deep 'foo' sound when you knock on it: foo -> フ, and the sound echoes through the raw wood.
knotty wood or tree