‘YHWH will cut off all flattering lips,
The tongue that makes great boasts,
Who have said, ‘With our tongue will we prevail,
Our lips are our own, who is lord over us?' '
YHWH assures him in his heart that it will not always be so. Those who have flattering lips will be cut off, as will those with a boastful tongue. They thought they could speak as they liked, they thought that their powerful words would enable them to achieve their own selfish ends, they challenged the right of anyone to be lord over them, they thought that none could gainsay them. But they will inevitably be proved wrong. They will discover that there is indeed One Who is Lord over them.
Very much in mind here are those in authority or seeking authority, those seeking to win people by half-truths and downright lies. Those seeking to get their own way by the power of speech. Thus they speak and think arrogantly, prior to their inevitable downfall.