Literal translation
millstones are falling
Meaning
it's raining extremely hard
Explanation
This expression means it's raining extremely hard, violently, and builds its image on a heavy, massive object to measure the intensity of the downpour. The pèiro de moulin, millstone, is one of the heaviest everyday objects in rural Provençal life, cut from hard rock to grind grain under its own weight: saying that such stones are falling from the sky amounts to describing rain of extraordinary violence and density, as if every drop struck the ground with the force and weight of a whole millstone. This hyperbole joins the same principle found in other Provençal and French weather expressions that measure the intensity of a natural phenomenon by the size or weight of a familiar object, a universal rhetorical device for making an abstract reality like rainfall intensity tangible. The register is familiar, used spontaneously during a particularly heavy storm, often as an immediate comment while watching the sky or listening to rain hammering the roofs. The expression carries no hidden figurative equivalent: it describes purely a weather phenomenon, with no shift of meaning into another domain.