Grass radical on top — three strokes evoking two sprouts side by side, the standing form of
艸. It supplies meaning:
薄 originally referred to thin reeds and slim plant stalks, then generalised to 'thin, slight, weak' — including
薄荷 (mint),
薄饼 (thin pancake) and
单薄 (flimsy). Same radical anchors a vast plant family.