Foolishness is behaviour that shows no sense. Sheer foolishness, put a stop to this foolishness, youthful foolishness.
It is milder than stupidity, because foolishness is about judgement rather than ability — clever people are perfectly capable of it. It is often used affectionately of children. Note fool, foolish and the -ness that makes the noun.
1nounfoolishnessBehaviour that shows no sense.
“I regret the foolishness of my youth, like driving far too fast.”
foolishness is a behaviour.
Foolish plus -ness, from fool, from Old French fol, from Latin follis, a bellows or windbag.
Middle English
folishnesse
English, today
foolishness
From Middle English folishnesse, follissnesse, equivalent to foolish + -ness.
Etymology from Wiktionary, CC BY-SA