foolish describes a person or an act showing a lack of good sense, judgment, or discretion, and in a lighter use simply what is absurd or ridiculous.
The word names a failure of judgment rather than a deficit of intelligence or information, which is what separates it from its usual substitutes. Stupid points at capacity, ignorant points at what someone has not been told, and foolish points at what someone did with what they already had — which is why a well-informed and quick-witted person can still act foolishly, and why the word attaches as readily to a single decision as to a character. Its milder sense drops the judgment entirely and means only silly or trifling, as in a foolish grin. The Oxford English Dictionary notes that the parent noun carries an implication of insulting contempt that its synonyms do not share to the same degree.
Foolish appears in the early fourteenth century as fool plus the suffix -ish. The noun is older, reaching English in the early thirteenth century from Old French fol, "madman, idiot," and behind it Medieval Latin follus and Latin follis, "bellows, leather bag." The path from an air-filled bag to an empty-headed person appears to run through Vulgar Latin, and the same image recurs independently in Sanskrit vatula-, "windy, inflated with wind." By the fourteenth century fool had also acquired its occupational sense of jester or court clown.
Foolishness is conventionally set against 📝wisdom, but the opposition is not always a ranking. 📝The Fool of the tarot inverts it, treating the state of knowing nothing as a clean slate and therefore as potential rather than defect — the same figure the licensed jester exploited, where speaking foolishly was the only safe way to speak truthfully.
"In examinations, the foolish ask questions that the wise cannot answer." ~ Oscar Wilde, Phrases and Philosophies for the Use of the Young, 1894