folly: the dull, thick-headed unwisdom that Ecclesiastes weighs against wisdom and finds worthless
This is a word that belongs to Ecclesiastes. All seven of its appearances sit in that one book, where the writer is testing what a human life is worth, and folly is the pole he keeps setting opposite wisdom. Proverbs treats foolishness mainly as a moral fault, a chosen refusal to fear God. Here it is closer to sheer dullness: the thick, unthinking way of living that gets a person nowhere.
The book uses it in two ways. In the chapters where the writer describes his search, folly is a category he gave his mind to examining, and it almost always travels in company with the word for madness (הוֹלֵלָה), the wild and unhinged side of unwisdom. He does not merely watch it from a distance; he says he took hold of it on purpose, to see what it would yield. His verdict is that wisdom beats it as plainly as light beats darkness.
Later in the book the word comes down to earth and describes actual people. A little of it, like a dead fly in the perfumer's oil, is enough to spoil a good name. And it is what a fool's talk is made of, from the first word out of his mouth.
Behind it stands a root that also gives Hebrew a word for the fool himself (סָכָל) and a second, rarer noun for folly (סֶכֶל). All of them point the same direction: not madness, and not wickedness exactly, but a stupidity that wastes a life.