Literal translation
you would make the flint fall off the rifle
Meaning
you really surprise me
Explanation
The expression conveys extreme astonishment at someone's slowness or clumsiness, to the point of losing patience with them. Literally, me fariés toumba la pèiro dóu fusiéu translates as you'd make the flint fall off the rifle: the image refers to the old flintlock rifle, whose flint, fixed in the weapon's hammer, produced the spark needed to ignite the powder. A sufficiently violent or startling jolt could dislodge that flint from its socket, hence the idea of a surprise so sharp it would shake loose an otherwise firmly fixed mechanism. Apparently used first to mock someone's exasperating slowness, the expression widened toward a broader sense of strong astonishment, close to the French tu m'étonnes. The register is familiar, often tinged with exasperation or irony, and the technical reference to the flintlock places the image in a rural and military vocabulary, now dated but well attested in older dictionaries.