to prove a fool: to do something that turns out to be folly, or to make someone else's wisdom turn out to be folly
This verb turns up only eight times in the Hebrew Bible, and six of those do the same job: they pin the word "fool" on one particular thing a person did.
- Samuel tells Saul he has acted foolishly by offering the sacrifice himself, and that his family will not keep the kingdom.
- Hanani the seer tells King Asa he has acted foolishly by buying off the king of Aram, and that he will have wars from now on.
- Laban throws it at Jacob for slipping away with his daughters instead of letting him kiss them goodbye.
- Saul says it of himself, after David spares his life in the camp at night.
- David says it of himself twice, once in Samuel and once in Chronicles, after counting his fighting men.
That is what sets this word apart from the wisdom books' vocabulary of folly, which mostly describes a type of person you could point out in a crowd. This one is a verdict on an act, passed after the fact, spoken either as a confession or as an accusation. And in Samuel and Chronicles what follows it is heavy: a lost dynasty, a lifetime of wars, a plague on the nation.
The other two occurrences run the opposite direction. Rather than someone acting foolishly, someone else's wisdom is turned into foolishness. David prays that God will do exactly that to the advice of Ahithophel, the sharpest counselor in the kingdom, and Isaiah says God does it to diviners and to men who trade in knowledge. The picture is not of a clever man being made stupid, but of his clever plan coming out looking like a fool's.
The same root gives Hebrew its name for the fool himself (סָכָל) and two of its words for folly (סִכְלוּת and סֶכֶל). One oddity is worth knowing: a nearly identical Hebrew verb (שָׂכַל) means close to the opposite, to act with insight and come out well.