The meaning is that “he, the king of Syria” (2 Kings 13:4 Hazael) limited the standing army of Jehoahaz.
Like the dust by threshing - An expression not only employed metaphorically, and importing defeat, conquest, and grinding oppression Jeremiah 51:33; Micah 4:12, but implying also the literal use of threshing-instruments in the execution of prisoners of war (marginal reference, and compare 2 Samuel 12:31).