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. A new sharp threshing instrument having teeth "A threshing wain; a new corn-drag armed with pointed teeth" - See note on Isaiah 28:27-28.
Thou shalt thresh the mountains - Mountains and hills are here used metaphorically for the kings and princes of the Gentiles. - Kimchi.