Behold, I will make thee a new sharp threshing instrument having teeth: thou shalt thresh the mountains, and beat them small, and shalt make the hills as chaff.
Behold, I will make thee a new sharp threshing instrument - God will make Israel to destroy their enemies as the Eastern grain-drag (Isaiah 28:27-28) bruises out the grain with its teeth, and gives the chaff to the winds to scatter.
Having teeth - serrated, so as to cut up the straw for fodder, and separate the grain from the chaff.
Thou shalt thresh the mountains ... hills - kingdoms more or less powerful that were hostile to Israel (Isaiah 2:14).