thick (grass)
丵 centers on thick, clustered growth, specifically of grass or vegetation. The idea is dense, tufted grass rather than a single blade.
丵 is a pictograph of thick grass growing in a tuft. The modern form retains the clustered vertical strokes suggesting dense vegetation.
The top 业 looks like a patch of ground, and the lower strokes are many blades of grass sprouting thickly together. Picture a dense tuft of grass that is hard to separate.
For サク, imagine a sack full of thick grass clippings: sack -> サク, and the sack is stuffed with dense, clustered grass.