They set their mouth against the heavens - Compare Revelation 13:6. Literally, “They set their mouth in heaven,” or in the heavens. The idea is, they speak as if they were “in” the heavens; as if they were clothed with all authority; as if they were superior beings, and had a right to command the universe.
And their tongue walketh through the earth - It has no limit; it is as if it roamed over all the earth. They speak without any restraint of law, or propriety; without any regard to the command of God, or to what is due to people, In other words, they seem to set themselves above all law, and to act as if there were no one in heaven or in earth to control them.