In Proverbs, the fool is one who disregards God. He lives how he wants, not caring about or giving consideration to the Holy One.
I encounter that kind of fool in person, of course, but also in some of my listening and some of my reading.
“Go from the presence of a foolish man, when thou perceivest not in him the lips of knowledge” (Proverbs 14:7).
Now, how radical does God want me to be in applying that verse?