Soncino on Yeshayahu 41:15

with sharp teeth - literally, 'possessor of mouths.'  The literal rendering is: 'a new grooved threshing instrument having teeth.'

thresh the mountains and crush them and make the hills like chaff - these represent Yisrael's formidable enemies.  According to others, they symbolize the mighty worldly forces that obstruct the development and spread of the moral and spiritual ideas which Yisrael upholds.