The beginning of the words of his mouth is foolishness. A confirmation of the last clause of the preceding verse. The fool speaks according to his nature. "As saith the proverb of the ancients, Out of the wicked cometh forth wickedness" (1 Samuel 24:13; cutup.
Proverbs 15:2; Isaiah 32:6). As soon as he opens his month he utters folly, unwisdom, silliness. But he does not stop there. The end of his talk is mischievous madness. By the time he has finished, he has committed himself to statements that are worse than silly, that are presumptuous, frenzied, indicative of mental and moral depravity.
Intemperate language about the secrets of God's providence and the moral government of the world may be intended. Some think that the writer is still alluding to dangerous talk concerning a tyrannical ruler, seditious proposals, secret conspiracies, etc.
The text itself does not confirm such notion with any certainty.